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;
     }

applications/utilities/mesh/advanced/modifyMesh/modifyMesh.C

@@ -413,7 +413,7 @@ int main(int argc, char *argv[])
     const SubList<face> outsideFaces
     (
         mesh.faces(),
-        mesh.nFaces() - mesh.nInternalFaces(),
+        mesh.nBoundaryFaces(),
         mesh.nInternalFaces()
     );