[test][parallelgrid] Dump ranks to VTK output

dune/grid/test/test-parallelgrid.cc

@@ -175,6 +175,8 @@ int main (int argc , char **argv)
   }
 
   Dune::VTKWriter<ParallelGridType::LeafGridView> vtkWriter(grid.leafGridView());
+  std::vector<int> ranks(grid.leafGridView().size(0), myrank);
+  vtkWriter.addCellData( ranks, "rank" );
   vtkWriter.write( "parallel_grid" );
 
   return 0;