Fix ViennaGrid assertion due to 'write_opendx' (this function hasn't been tried, but it was implemented the same way as 'write_vtk' and must thus also exhibit the same error).
Fix ViennaGrid assertion due to 'write_opendx' (this function hasn't been tried, but it was implemented the same way as 'write_vtk' and must thus also exhibit the same error).

jonancm/viennagrid-python,jonancm/viennagrid-python,jonancm/viennagrid-python