- reduced clutter when iterating over containers

- reduced clutter when iterating over containers

- delay dereferencing of optional subRegion entries until an
  objectRegistry is required.

  This improves usabilty when reference objects do not yet exist
  at the time of construction.

- complete any pending initialisation on write().
  Allows lazier evaluation until when surfaces are actually available.

- complete any pending initialisation on write().
  Allows lazier evaluation until when the surfaces are actually needed.

- includes initial relocation of low-level vtu handling.
  Extracted, refactored from the Catalyst function object.

- reduced clutter when iterating over containers

- reduced clutter when iterating over containers

- reduced clutter when iterating over containers

- reduced clutter when iterating over containers

- reduced clutter when iterating over containers

- previously it was only possible to reset a pointer, but not to
  change a const-reference directly (needed a swap() to do this).

- currently supports pTotal, rhoU.

- makes it easier to implement functionObject or field object removal

- use registry checkOut for removeRegisteredObject function object

The occurrence is from cells with vertices that are shared between two faces
only (these vertices can originate from hex refinement). Decomposing both faces
can occasionally produce triangles with identical vertices and this results in a
non-manifold edge which triggers the erosion procedure.

Avoided by detecting cells with these special vertices and making sure the tet-decomposition
never uses the same points on the faces using them.

Patch contributed by Mattijs Janssens

- 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