- not allowed in C++20

COMP: avoid hidden overloaded-virtual (edgeMesh, surfMesh)

Use MPI Get_elements_x() for message sizes

See merge request !681

- ensures more accurate values for message sizes than using
  MPI Get_count(), which trucates at INT_MAX

- add more/better error messages when trying to receive messages
  that exceed INT_MAX or the char buffer lengths

- PstreamBuffers are nonBlocking by default, so no need to re-specify

- "buffered" corresponds to MPI_Bsend (buffered send),
  whereas the old name "blocking" is misleading since the
  regular MPI_Send also blocks until completion
  (ie, buffer can be reused).

ENH: IPstream::read() returns std::streamsize instead of label (#3152)

- previously returned a 'label' but std::streamsize is consistent with
  the input parameter and will help with later adjustments.

- use <label> instead of <int> for internal accounting of the message
  size, for consistency with the underyling List<char> buffers used.

- improve handling for corner case of IPstream receive with
  non-blocking, although this combination is not used anywhere

Fix fvsPatchFields reading/writing the "value" entry

See merge request !679

- robuster and more reliable determination of inserted objects
  (previous code could produce false positives).

  Now also determine the number of inserted objects within the
  constructor (instead of simply storing a bool). This allows reuse
  in the address calculations to reduce overheads there.

BUG: dodgy short-circuit logic for insertedObjectLabels()

- as a quick short-circuit it previously created as demand-driven
  pointer with a zero elements. However, this meant that if this code
  was called first (before any other addressing), subsequent calls to
  the addressing would fail.

BUG: bad logic and lookup for faAreaMapper (#3147)

- was using labelHashSet but returning a bool and set using the
  addressing using the unfiltered objects!
  This is a latent bug since interpolated/inserted faces not
  currently supported anyhow.

ENH: use std::unique_ptr for demand-driven data

- manual revert for 44d0fbd5.

  Unless required for post-processing the "value" entry should not
  written if it is not mandatory for reading.

  This is especially crucial with an 'empty' patch field where the
  field (size 0) has a different size from the patch.

- the changes made align fvsPatchField behaviour with fvPatchField

- return autoPtr<token::compound> instead of the derived type,
  otherwise cannot easily construct a token from it

ENH: additional typed version of refCompoundToken()

- symmetric with typed version of transferCompoundToken()
  and isCompound()

- add ITstream::findCompound<Type>() method.
  Useful for searching within token streams

reduce communication argList at startup

See merge request !678

- previously automatically skipped the first communicator (which was
  assumed to be MPI_COMM_WORLD), but now simply rely on the
  internal pendingMPIFree_ to track which communicators have actually
  been allocated.

- UPstream::Communicator is similar to UPstream::Request to
  wrap/unwrap MPI_Comm. Provides a 'lookup' method to transcribe
  the internal OpenFOAM communicator tracking to the opaque wrapped
  version.

- provide an 'openfoam_mpi.H' interfacing file, which includes
  the <mpi.h> as well as casting routines.

  Example (caution: ugly!)

     MPI_Comm myComm =
         PstreamUtils::Cast::to_mpi
         (
             UPstream::Communicator::lookup(UPstream::worldComm)
         );

- replace point-to-point transmission of the argList args/options with
  a broadcast. This is sufficient for most cases (without distributed
  roots).

  For "normal" cases (non-distributed roots) this will replace the
  nProcs-1 messages with a single broadcast.

- for cases with distributed roots, distinguish between a single,
  identical root and different roots.  An identical root can also be
  subsequently handled with a broadcast. Different roots will still
  require individual point-to-point communication.

  For cases with distributed roots, it will add the overhead of an
  additional broadcast.

- adds consistency with regular point-to-point parallel streams.

ENH: add ITstream::tokens() methods

- useful, for example, to disambiguate constructing tokenList from
  a ITstream.

  ITstream& is = otherDict.lookup("original");

  tokenList deepCopy(is);  // Error: from Istream or tokenList??
  tokenList deepCopy(is.tokens());  // OK: from tokenList

- same as append() did, but with more consistent names

STYLE: replace special purpose append() with container method

STYLE: update code style

update to internal accounting for finite-area

See merge request Development/openfoam!675

- The internal storage location of finite-area changes from being
  piggybacked on the polyMesh registry to a having its own dedicated
  registry:

  * allows a clearer separation of field types without name clashes.
  * prerequisite for supporting multiple finite-area regions (future)

Old Locations:
```
   0/Us
   constant/faMesh
   system/faMeshDefinition
   system/faSchemes
   system/faSolution
```

New Locations:
```
   0/finite-area/Us
   constant/finite-area/faMesh
   system/finite-area/faMeshDefinition  (or system/faMeshDefinition)
   system/finite-area/faSchemes
   system/finite-area/faSolution
```

NOTES:
    The new locations represent a hard change (breaking change) that
    is normally to be avoided, but seamless compatibility handling
    within the code was found to be unworkable.

    The `foamUpgradeFiniteArea` script provides assistance with migration.

    As a convenience, the system/faMeshDefinition location continues
 ...

- was using blocking (=buffered) comms
- was running out of MPI_COMM_SIZE buffer space
- now using PstreamBuffers to have non-blocing comms

- lower memory overhead, consistent access pattern

- use an internal 'typedef MeshObject<...> MeshObject_type' within
  derived classes. Reduces clutter and eases any updates.

- Delete() will perform a 'checkOut()' which does the following:
  * remove the object from the registry
  * delete the pointer (if owned by the registry)

- Release() does the following:
  * transfer ownership of the pointer (if owned by the registry)

- Store() does the following:
  * transfer ownership of the pointer to the registry

ENH: use UPtrList of sorted objects for MeshObject updates

- few allocations and lower overhead than using a HashTable,
  ensures the same walk order over the objects (in parallel)

STYLE: adjust meshObject debug statements

STYLE: update code style for phi modification (engine motion)

ENH: pass isMeshUpdate param in fvMesh/polyMesh clearOut() methods

- top-level use of isMeshUpdate parameter to clearOut and
  clearAddressing was being inadvertently filtered out

- after the modification of d578d48a, the parent was now actually
  searched. However, should be returning "constant" and not trigger a
  FatalError if the file/directory is not found.

- findStrings, findMatchingStrings now mostly covered by matching
  intrinsics in wordRe and wordRes.

  Add static wordRes match() and matching() variants

COMP: remove stringListOps include from objectRegistry.H

- was already noted for removal (NOV-2018)

- an example of the new, more succinct refConstCast version:

      auto& abc = refConstCast<adjointVectorBoundaryCondition>(Uab);

  older:
      adjointVectorBoundaryCondition& abc =
          refCast<adjointVectorBoundaryCondition>
          (
              const_cast<fvPatchVectorField&>(Uab)
          );
  or:
      adjointVectorBoundaryCondition& abc =
          const_cast<adjointVectorBoundaryCondition&>
          (
              refCast<const adjointVectorBoundaryCondition>(Uab)
          );

- an example of the new, more succinct isA_constCast version:

      auto* acapPtr = isA_constCast<fieldType>(abf[patchi]);

      if (acapPtr)
      {
          auto& acap = *acapPtr;
          ...
      }

  older:
      if (isA<fieldType>(abf[patchi]))
      {
          fieldType& acap =
              const_cast<fieldType&>
              (
                  refCast<const fieldType>(abf[patchi])
              );
          ...
      }

STYLE: remove spurious 'const' qualifier from isA<> use

- NewIFstream would read complete remote file to decide if
  was collated.
- This limits files to 31bit size
- Instead now have master-only opening of file.
- Still has problem with refinement history/cellLevel etc.

- previously would always return "constant" as the instance for
  an optional dir/file that wasn't found.
  However, this meant retesting to screen out false positives.
  Now support an additional parameter
      'bool constant_fallback = ...'
  to return "constant" or an empty word.

  The method signature changes slightly with a new optional bool
  parameter:

      //! Return \c "constant" instead of \c "" if the search failed
      const bool constant_fallback = true

ENH: code consolidation for findInstancePath

- relocate from Time to TimePaths and provide an additional static
  version that is reused in fileOperations

BUG: distributedTriSurfaceMesh:::findLocalInstance broken (#3135)

- was not checking the parent at all.

COMP: remove unused findInstancePath(const fileName&, ..) method

- support construct faceZoneSet from faceSet (#3126)

ENH: support unregistered loading of cell/face/point sets

- leave 'readContents' method name for exposed (public) methods.
  Generally not a problem, but can confuse the compiler when various
  public/private versions are available with the same number of
  parameters.

STYLE: adjust meshObject debug statements

- regIOobject::store(std::unique_ptr<...>&& ptr)
- IOobjectList::add(std::unique_ptr<...>&& ptr)

STYLE: io.globalCaseName() instead of io.time().globalCaseName() [#3007]

ENH: combine broadcasts for uncollated master-only handling