clear 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
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