- this makes it easier to split creation into a two-stage process
  as required

- extend handling for polyBoundaryMeshEntries, faBoundaryMeshEntries
  with more functionality. Ensure that these are never registered.

ENH: addition writeEntry methods for polyBoundaryMesh

- simplifies streaming and collating into other files

ENH: polyMesh rereading - update owner/neighbour header information

- this avoids accidentally reading the "cells" file if the mesh has
  been created with NO_READ and then updated

STYLE: less vertical space when outputting empty PtrList