fully remove and clean process registration and h5 file link mecanism

• process registration was used to keep trace of each processes and dump results. This will be done at another level in the future (probably by ewosk)
  • remove Process.register_process function (and call to it)
  • remove _NexusDatasetDef class which was used to define which field must be saved for a process
• remove saving_point widget: was using the regitered processes and needed for large number of spectrum. This is not the use case today and if we want to have such a mecanism it will be done another way in the future.
• remove write_process parameter from XASWriter.dump_xas
• XASObject:
  • remove keep_process_flow from constructor
  • remove link_to_h5, has_linked_file, get_next_processing_index, register_processing, get_process_flow, clean_process_flow and copy_process_flow_to functions
  • to_dict: remove with_process_details parameter and link mecanism
• update unit test