turn dipole attrs into properties

[sappelhoff]

• closes mne.Dipole attributes not documented in API docs #13150

To do

• update dipole docs with info from doc: point to read_dipole #13149 (comment)

[drammock]

Lots of CI errors of the form:

FAILED mne/beamformer/tests/test_rap_music.py::test_rap_music_simulated - AttributeError: property 'pos' of 'Dipole' object has no setter

Normally how we handle this is to change the previously quasi-public attributes into private ones, e.g., in the class init and throughout the codebase we set Dipole._pos (note the underscore) and use the non-private one when we just need to get the value. If we expect / want users to also be able to set the value of any of these properties (name maybe?) then we would need to add a second public def decorated with @setter, e.g.

@setter
def name(self, name):
    assert isinstance(name, str)  # optional validation, could also use `_validate_type`
    self._name = name

Fixed in latest commits

[larsoner]

Marking for merge-when-green, thanks in advance @sappelhoff !