Move xas_obj to hdf5 container

All the treatment should be stored in a hdf5 with one NXProcess for each treatment. With input, output, order and configuration of the process. It should at least be an option. We could have something like

xas_obj.link_to_h5(h5file)

If a XASObject is link to an hdf5 file then the process function should store input, output as NXProcess.