vtkUnstructuredGridToExplicitStructuredGrid is not parallel friendly
This illustrated by failure in the test
pvcs.ExplicitStructuredGridReal when running with MPI. The issue is that
vtkUnstructuredGridToExplicitStructuredGrid is given a non-empty UG only on root node (since input dataset in vtu file, which is only read on rank 0). Subsequently, vtkUnstructuredGridToExplicitStructuredGrid produces bogus grid on rank > 0 and that causes the
vtkExplicitStructuredGridSurfaceFilter to raise errors down the pipe. The test has been disabled for now.