vtkDistributedDataFilter is a serial implementation that acts as a
stand-in for vtkPDistributedDataFilter when MPI suppport is not
available. It is nearly a pass-through filter, except that it copies
non-composite inputs into vtkUnstructuredGrids to be consistent with
the output produced by vtkPDistributedDataFilter. Member functions in
the public API have no effect on the execution of this filter.

Made vtkPDistributedDataFilter a subclass of vtkDistributedDataFilter