ENH: support vtu mesh subsetting, creation from cellShapes

- support simple mesh subsetting to vtu formats to enable debug output
  for a subsection of an existing polyMesh

- rudimentary support for VTK from cellShapes is intended for handling
  basic primitive cell shapes without a full blown polyMesh description.

  For example,

     // Create an empty polyMesh with points only
     polyMesh debugMesh
     (
         io,
         std::move(points),
         faceList(),     // faces
         labelList(),    // owner
         labelList(),    // neighbour
         true            // syncPar
     );

    // Establish the appropriate VTK sizing for the TET shapes
    vtk::vtuCells vtuCells;
    vtuCells.resetShapes(debugCutTets);
    vtuCells.nPoints(debugMesh.nPoints());

  NOTE
  Since the vtk::internalMeshWriter only uses the polyMesh reference
  for the points, it is also possible to create the vtuCells
  description without a pointField (or from a different mesh
  description) and write out the connectivity using the pointField
  from a different mesh.

applications/test/foamMeshToTet-vtk/Make/files

@@ -0,0 +1,4 @@
+foamMeshToTet-vtk.C
+writeVTKtetMesh.C
+
+EXE = $(FOAM_USER_APPBIN)/foamMeshToTet-vtk

applications/test/foamMeshToTet-vtk/Make/options

@@ -0,0 +1,7 @@
+EXE_INC = \
+    -I$(LIB_SRC)/fileFormats/lnInclude \
+    -I$(LIB_SRC)/meshTools/lnInclude
+
+EXE_LIBS = \
+    -lfileFormats \
+    -lmeshTools