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/preProcessing/faceAgglomerate/faceAgglomerate.C

@@ -128,7 +128,7 @@ int main(int argc, char *argv[])
     }
 
     // Sync agglomeration across coupled patches
-    labelList nbrAgglom(mesh.nFaces() - mesh.nInternalFaces(), -1);
+    labelList nbrAgglom(mesh.nBoundaryFaces(), -1);
 
     forAll(boundary, patchi)
     {

applications/utilities/preProcessing/setFields/setFields.C

@@ -239,7 +239,7 @@ bool setFaceFieldType
         const Type& value = pTraits<Type>(fieldValueStream);
 
         // Create flat list of selected faces and their value.
-        Field<Type> allBoundaryValues(mesh.nFaces()-mesh.nInternalFaces());
+        Field<Type> allBoundaryValues(mesh.nBoundaryFaces());
         forAll(field.boundaryField(), patchi)
         {
             SubField<Type>
@@ -447,7 +447,7 @@ int main(int argc, char *argv[])
             (
                 mesh,
                 "faceSet",
-                (mesh.nFaces()-mesh.nInternalFaces())/10+1
+                mesh.nBoundaryFaces()/10+1
             );
 
             source->applyToSet

applications/utilities/preProcessing/viewFactorsGen/viewFactorsGen.C

@@ -82,10 +82,7 @@ triSurface triangulate
     const polyMesh& mesh = bMesh.mesh();
 
     // Storage for surfaceMesh. Size estimate.
-    DynamicList<labelledTri> triangles
-    (
-        mesh.nFaces() - mesh.nInternalFaces()
-    );
+    DynamicList<labelledTri> triangles(mesh.nBoundaryFaces());
 
     label newPatchI = 0;
     label localTriFaceI = 0;