The ghost cells generator, which used to only work for
vtkUnstructuredGrid, is now replaced by the new ghost cells generator
that works on vtkImageData, vtkRectilinearGrid, vtkStructuredGrid,
vtkUnstructuredGrid, and any vtkMultiBlockDataSet,
vtkPartitionedDataSet, and vtkPartitionedDataSetCollection composed of
the latter base types.

The old backend is being removed.