ENH: add boundary halo handling to faMesh

- the finiteArea is typically restricteed to one or more patches on a
  polyMesh but will have an external connectivity to other parts of
  the polyMesh. For proper determination of the point normals (for
  example), the neighbouring information is needed.

  These outside 'halo' faces can be located on different processors,
  but not correspond to an internal processor-processor interface.

  Add a dedicated form of mapDistribute for swapping this type of
  information. Since this is a collective operation, locate
  corresponding methods directly on the faMesh level instead of
  the faPatch level.