[vtk-hdf] test struct grids with non-axis aligned orientation

[dglaeser]

The two tests test_vtk_hdf_image_grid_writer.cpp and test_vtk_hdf_parallel_image_grid_writer.cpp currently don't regression test the written files with non-axis-aligned orientation. That is because the way we read the grid points (using vtk) in the regression test script does not yield the correct coordinates. Interestingly, ParaView displays the written files as expected. Moreover, we use the same functions to extract the points from grids read with the vtkImageGridReader, which also works fine...