Implement unpack_self() for MultiParameter

[thangleiter]

This PR implements the unpack_self() method for MultiParameter, extending #7346. This makes the special treatment of this type of parameter in DataSaver.add_result() redundant and should allow users to make use of the new has_control_of and is_controlled_by features in custom MultiParameter classes.

To Do

• [ ] modify DataSet._register_multiparameter() to account for has_control_of and is_controlled_by attributes.