ENH: simple faMeshSubset (zero-sized meshes only)

ENH: additional access methods for faMesh, primitive geometry mode

- wrapped walking of boundary edgeLabels as list of list
  (similar to edgeFaces).

- primitive finiteArea geometry mode with reduced communication:
  primarily interesting for decomposition/redistribution (#2436)

ENH: extra vtk debug outputs for checkFaMesh

- report per-processor sizes in the mesh summary