- move declaration of emptyLabelList (now deprecated) out of List.H

- can now safely use labelList::null() instead of emptyLabelList for
  return values. No special treatment required for lists.

  Possible replacements:

      if (notNull(list) && list.size())  ->  if (list.size())
      if (isNull(list) || list.empty())  ->  if (list.empty())

  The receiver may still wish to handle differently to distinguish
  between a null list and an empty list, but no additional special
  protection is required when obtaining sizes, traversing, outputting
  etc.

- relocate zone IO from Detail::MeshedSurfaceIOAllocator into surfMesh
  directly to allow re-purposing of MeshedSurfaceIOAllocator

- provide meshedSurf::emptySurface zero-sized placeholder implementation

- add concrete implementation of meshedSurf::zoneIds() to simplify
  overloading

- previously had a single pointer/value zeros (8 bytes), this meant
  that the reinterpret cast to a List would yield a reference that
  could be unsafe under certain conditions.

  Eg,
     const labelList& myList = labelList::null();

     Info<< myList.size() << nl; // OK since size is the first parameter

     SubList<label>(myList, 0);  // Unsafe

  The SubList usage is unsafe since it passes in pointer and size into
  the underlying UList. However, the pointer from the labelList::null()
  will be whatever happens to be around in memory immediately after the
  NullObject singleton. This is mostly not a problem if the List size
  is always checked, but does mean that the data pointer is rather
  dubious.

- Increase the size of the nullObject singleton to 32 bytes of zeros
  to ensure that most reinterpret casting will not result in objects
  that reference arbitrary memory.

  The 32-byte data size is rather arbitrary, but covers most basic
  containers.

Feature object registry search

See merge request OpenFOAM-plus!232

  - objectRegistry search, erase methods
  - clip, minMax
  - function object triggering
  ...

- The findObject() methods are template-typed and used to locate a
  particular Type/name combination.

  Eg,

      volScalarField* ptr = obr.findObject<volScalarField>("xyz");

- The findIOobject() methods are un-typed and use the name only.

  Eg,

      regIOobject* ptr = obr.findIOobject("xyz");

The typed versions will be most commonly used, but the un-typed lookup
can be useful in a templating.

- Simplified findObject* methods to use findIOobject* as the backend.

- similar to what erase() does, but as a mutable operation (#1180)

- replace basicThermo lookupAndCheckout (commit 880c9875) with
  the new objectRegistry::checkOut() method.

- implemented as lazy evaluation with an additional update() method.
  This avoids unnecessary changes until the values are actually
  required.

- apply mesh motion changes for momentum, volFieldValue,
  specieReactionRates function objects

- forwards to the underlying HashTable erase, but frees things owned
  by the registry as well (ie, performs a checkOut)

- also removed some editing cruft from controlDict that should not
  have been there

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=56480

from e to T. T proved to be more generic solution.

Add minMax and clip field functions

See merge request OpenFOAM-plus!230

STYLE: avoid local variable name masking

- changed ensightOutput from a class solely comprising static methods to
  a namespace and added in sub-namespaces Detail and Serial.

  This makes it easier to "mix-in" functions at different levels.
  Refactored and combined some serial/parallel code where possible.

  The general ensightOutput namespace has now shifted to be in the
  fileFormats lib, while leaving volField outputs in the conversion lib
  and cloud outputs in the lagrangian-intermediate lib.

  The ensightCloud namespace is now simply folded into the new
  ensightOutput namespace.

  These changes clean up some code, reduce fragmentation and
  duplication and removes the previous libconversion dependency for
  sampling.

- use int for ensight nTypes constexpr

Note: issue #1176 is unaffected except for the change in file name:

   ensightOutputTemplates.C -> ensightOutputVolFieldTemplates.C

Function object updates

See merge request OpenFOAM-plus!231