STYLE: add nBoundaryFaces() method to primitiveMesh

- nBoundaryFaces() is often used and is identical to
  (nFaces() - nInternalFaces()).

- forward the mesh nInternalFaces() and nBoundaryFaces() to
  polyBoundaryMesh as nFaces() and start() respectively,
  for use when operating on a polyBoundaryMesh.

STYLE:

- use identity() function with starting offset when creating boundary maps.

     labelList map
     (
         identity(mesh.nBoundaryFaces(), mesh.nInternalFaces())
     );

  vs.

     labelList map(mesh.nBoundaryFaces());
     forAll(map, i)
     {
         map[i] = mesh.nInternalFaces() + i;
     }