ENH: multiple zone selection for fvMeshSubsetProxy (#973)

- handle tmp fields in interpolate methods

- special method interpolateInternal() for creating a volume field
  with zero-gradient treatment for patches from an internal field.

  This method was previously also called interpolate(), but that
  masked the ability to subset the internal field only.

  Ensight output needs the volume field:
      uses interpolateInternal().

  VTK output has separate handling of internal and patch fields:
      uses interpolate().

ENH: added fvMeshSubset mesh() method for baseMesh or subMesh.

- simplies coding when the fvMeshSubset may or may not be in active use.

ENH: update foamToEnsight to use newer methods in wrapped form

- static interpolate functions with renaming for manual use with
  fvMeshSubset (when fvMeshSubsetProxy may be too limiting in
  functionality)