ENH: better handling of various face types in MeshedSurface (issue #294)

- ensure that MeshedSurface and UnsortedMeshedSurface can also work
  with labelledTri faces.

- nTriangles() convenience method for meshed surfaces

- MeshedSurface::addZonesToFaces() method to copy zone information
  into the labelledTri per-face region storage.
  The goal is to replace the triSurface reading routines with
  run-time selectable ones from surfMesh.