This provides a generic method for traversing cells in
subclasses of vtkDataSet. Three implementations of the
vtkCellIterator interface are provided:

* vtkDataSetCellIterator: Use vtkDataSet API to look up cells
  using random access.
* vtkPointSetCellIterator: Use the vtkPointSet API to look up
  cells using random access. Advantage over
  vtkDataSetCellIterator is that points are looked up as a batch
  operation.
* vtkUnstructuredGridCellIterator: Use the vtkUnstructuredGrid
  internals to perform efficient sequential iteration without
  vtkCellLinks.

Change-Id: I5893dab945115721d2291b2eff83bf7e71632230