- delay construction of message buffer

- OStringStream count() method to test if anything has been streamed

STYLE: explicit use of std::ios_base in IOstreams

- document the return information of set flag methods

- can be used with this type of code:

  ITstream* streamPtr = dict.findStream(name);
  if (streamPtr)
  {
      auto& is = *streamPtr;
      ...
  }

  versus:

  const entry* eptr = dict.findEntry(name);
  if (eptr && eptr->isStream())
  {
      auto& is = eptr->stream();
      ...
  }

ENH: add findStream(), streamPtr(), isStream() to dictionary search

- symmetric with findDict(), dictPtr(), isDict() methods

STYLE: use findDict() instead of found() + subDict() pairing

COMP: define is_globalIOobject trait at top of IOobject header

- more visibility, permits reuse for specializations etc.

BUG: streamFunction used uninitialized values for symmetry patches

- related to 8a8b5db9 changes (#3144)

ENH: improve robustness of surface field flattening

- vtk::surfaceFieldWriter

- simplifies forward declarations

- the PATH on WSL will usually contain spaces, so it needs quoting

ENH: improve handling of multi-pass send/recv

See merge request Development/openfoam!680

- the maxCommsSize variable is used to 'chunk' large data transfers
  (eg, with PstreamBuffers) into a multi-pass send/recv sequence.

  The send/recv windows for chunk-wise transfers:

      iter    data window
      ----    -----------
      0       [0, 1*chunk]
      1       [1*chunk, 2*chunk]
      2       [2*chunk, 3*chunk]
      ...

  Since we mostly send/recv in bytes, the current internal limit
  for MPI counts (INT_MAX) can be hit rather quickly.

  The chunking limit should thus also be INT_MAX, but since it is
  rather tedious to specify such large numbers, can instead use

      maxCommsSize = -1

  to specify (INT_MAX-1) as the limit.
  The default value of maxCommsSize = 0 (ie, no chunking).

Note
~~~~
  In previous versions, the number of chunks was determined by the
  sender sizes. This required an additional MPI_Allreduce to establish
  an overall consistent number of chunks to walk. This additional
  overhead each time meant that max...

- avoids polluting standard output for utilities such as
  foamDictionary etc

- the timeSelector is often used to select single or multiple times
  (eg, for post-processing). However, there are a few applications
  where only a *single* time should be selected and set.

  These are now covered by this type of use:

      timeSelector::addOptions_singleTime();  // Single-time options
      ...
      // Allow override of time from specified time options, or no-op
      timeSelector::setTimeIfPresent(runTime, args);

   In some cases, if can be desirable to force starting from the
   initial Time=0 when no time options have been specified:

      // Set time from specified time options, or force start from Time=0
      timeSelector::setTimeIfPresent(runTime, args, true);

   These changes make a number of includes redundant:

     * addTimeOptions.H
     * checkConstantOption.H
     * checkTimeOption.H
     * checkTimeOptions.H
     * checkTimeOptionsNoConstant.H

ENH: add time handling to setFields, setAlphaField (#3143)

    Co-authored-...

- for example,

     string buffer = ...;
     SubStrings<string> split;
     {
         auto colon = buffer.find(':');

         if (colon != std::string::npos)
         {
             split = stringOps::splitSpace(buffer, colon+1);
         }
     }

    Not really possible with a substr() since that would create a new
    temporary which then disappears.  Similarly awkward to split and
    then scan for the ':' to decide how many to discard.

ENH: add pop_front() and pop_back() methods to SubStrings

- the content is trivial enough (a pair of iterators) and the total
  number of elements is usually reasonable short so that removal of
  elements is inexpensive

  For example,

     string buffer = ...;
     auto split = stringOps::splitSpace(buffer);

     if (!split.empty() && split[0].str() == "face")
     {
         split.pop_front();
     }

- not allowed in C++20

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

- requires a field size check when copying into the flat boundary

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 Development/openfoam!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 Development/openfoam!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