ENH: relocate/refactor fvMeshSubset

- direct construct and reset method for creating a zero-sized (dummy)
  subMesh. Has no exposed faces and no parallel synchronization
  required.

- core mapping (interpolate) functionality with direct handling
  of subsetting in fvMeshSubset (src/finiteVolume).
  Does not use dynamicMesh topology changes

- two-step subsetting as fvMeshSubsetter (src/dynamicMesh).
  Does use dynamicMesh topology changes.
  This is apparently only needed by the subsetMesh application itself.

DEFEATURE: remove deprecated setLargeCellSubset() method

- was deprecated JUL-2018, now removed (see issue #951)