ENH: add fvPatch::offset() as pass-through to polyPatch::offset()

- makes fvPatch consistent with faPatch etc and can simplify coding

ENH: handle sizing within extrapolateInternal() patch field methods

- reduces reliance on any sub-components

ENH: extend polyBoundaryMesh patch selectors

- indices_if() method with a predicate, or with a patch-type
  behaves similarly to findPatchIDs(), but returns a sorted
  labelList instead of a labelHashSet

- nFaces_if() method with a predicate return the number of boundary
  faces for patches matching the given predicate.