VTKHDFWriter: support writing timesteps and partitions externally

Add support for 2 new options in vtkHDFWriter, based on the same principle: UseExternalPartitions and UseExternalPartitions. The first one writes different partitions in different files, and the second one does the same for timesteps. Then, subfiles are referenced by the main one using virtual datasets, which do not copy the data but only links it. This allows for standalone partitions or data for 1 time step, the same way !11101 (merged) did for composite types.