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/meshToMesh/meshToMesh.C

@@ -492,7 +492,7 @@ void Foam::meshToMesh::calculate(const word& methodName, const bool normalise)
         patches[0] = new polyPatch
         (
             "defaultFaces",
-            newTgt.nFaces() - newTgt.nInternalFaces(),
+            newTgt.nBoundaryFaces(),
             newTgt.nInternalFaces(),
             0,
             newTgt.boundaryMesh(),

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)
             {

src/sampling/sampledSurface/sampledTriSurfaceMesh/sampledTriSurfaceMesh.C

@@ -116,7 +116,7 @@ Foam::sampledTriSurfaceMesh::nonCoupledboundaryTree() const
         // all non-coupled boundary faces (not just walls)
         const polyBoundaryMesh& patches = mesh().boundaryMesh();
 
-        labelList bndFaces(mesh().nFaces()-mesh().nInternalFaces());
+        labelList bndFaces(patches.nFaces());
         label bndI = 0;
         for (const polyPatch& pp : patches)
         {

src/sampling/sampledSurface/sampledTriSurfaceMesh/sampledTriSurfaceMeshTemplates.C

@@ -58,7 +58,7 @@ Foam::sampledTriSurfaceMesh::sampleOnFaces
     auto& values = tvalues.ref();
 
     const polyBoundaryMesh& pbm = mesh().boundaryMesh();
-    const label nBnd = mesh().nFaces()-mesh().nInternalFaces();
+    const label nBnd = mesh().nBoundaryFaces();
 
     // Create flat boundary field
 

src/sampling/surface/isoSurface/isoSurface.C

@@ -1453,7 +1453,7 @@ Foam::isoSurface::isoSurface
 
 
     // Pre-calculate patch-per-face to avoid whichPatch call.
-    labelList boundaryRegion(mesh_.nFaces()-mesh_.nInternalFaces());
+    labelList boundaryRegion(mesh_.nBoundaryFaces());
 
     forAll(patches, patchi)
     {

src/sampling/surface/isoSurface/isoSurfaceTemplates.C

@@ -606,7 +606,7 @@ void Foam::isoSurface::generateTriPoints
 
 
     // Determine neighbouring snap status
-    boolList neiSnapped(mesh_.nFaces()-mesh_.nInternalFaces(), false);
+    boolList neiSnapped(mesh_.nBoundaryFaces(), false);
     List<Type> neiSnappedPoint(neiSnapped.size(), Type(Zero));
     for (const polyPatch& pp : patches)
     {

src/sampling/surface/triSurfaceMesh/discreteSurface.C

@@ -110,7 +110,7 @@ Foam::discreteSurface::nonCoupledboundaryTree() const
         // all non-coupled boundary faces (not just walls)
         const polyBoundaryMesh& patches = mesh().boundaryMesh();
 
-        labelList bndFaces(mesh().nFaces()-mesh().nInternalFaces());
+        labelList bndFaces(patches.nFaces());
         label bndI = 0;
         for (const polyPatch& pp : patches)
         {

src/sampling/surface/triSurfaceMesh/discreteSurfaceTemplates.C

@@ -169,7 +169,7 @@ Foam::discreteSurface::sampleOnFaces
         // Sample boundary faces
 
         const polyBoundaryMesh& pbm = mesh().boundaryMesh();
-        const label nBnd = mesh().nFaces()-mesh().nInternalFaces();
+        const label nBnd = mesh().nBoundaryFaces();
 
         // Create flat boundary field