- 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 connec...