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;
     }

src/OpenFOAM/meshes/polyMesh/globalMeshData/globalMeshData.C

@@ -1325,7 +1325,7 @@ void Foam::globalMeshData::calcGlobalPointBoundaryFaces() const
     // Global indices for boundary faces
     globalBoundaryFaceNumberingPtr_.reset
     (
-        new globalIndex(mesh_.nFaces()-mesh_.nInternalFaces())
+        new globalIndex(mesh_.nBoundaryFaces())
     );
     globalIndex& globalIndices = globalBoundaryFaceNumberingPtr_();