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/sampling/sampledSet/shortestPath/shortestPathSet.C

@@ -238,7 +238,7 @@ void Foam::shortestPathSet::genSamples(const polyMesh& mesh)
 
             // Situation 2: we're on a coupled patch and might need to
             //              switch processor/cell
-            boolList isFront(mesh.nFaces()-mesh.nInternalFaces(), false);
+            boolList isFront(mesh.nBoundaryFaces(), false);
 
             if (frontFaceI != -1)
             {