ENH: improved bookkeeping for finite-area to volume mesh correspondence

- whichPolyPatches() = the polyPatches related to the areaMesh.

  This helps when pre-calculating (and caching) any patch-specific
  content.

- whichPatchFaces() = the poly-patch/patch-face for each of the faceLabels.

  This allows more convenient lookups and, since the list is cached on
  the area mesh, reduces the number of calls to whichPatch() etc.

- whichFace() = the area-face corresponding to the given mesh-face

ENH: more flexible/consistent volume->area mapper functions