DOC: improve glossary entry about fiducials

[sappelhoff]

The glossary entry was a bit misleading due to its brevity:

1. there is no limit to fiducial points (i.e., there can be more than 3)
2. LPA, RPA, and NAS are anatomical landmarks, but "fiducials" is often used as a synonymous term for that

Potential links to add:

• https://en.wikipedia.org/wiki/Fiducial_marker
• https://www.fieldtriptoolbox.org/faq/how_should_i_report_the_positions_of_the_fiducial_points_on_the_head/
• bids disambiguation of anatomical landmarks and fiducials

[sappelhoff]

Also, this example part

mne-python/examples/visualization/plot_eeglab_head_sphere.py

Lines 62 to 64 in 5feaee8

	check_ch = ['Oz', 'Fpz', 'T7', 'T8']
	ch_idx = [fake_evoked.ch_names.index(ch) for ch in check_ch]
	pos = np.stack([fake_evoked.info['chs'][idx]['loc'][:3] for idx in ch_idx])

should probably be replaced by using this function, right?

mne-python/mne/channels/montage.py

Lines 308 to 349 in 972976f

	def get_positions(self):
	"""Get all channel and fiducial positions.
	Returns
	-------
	positions : dict
	A dictionary of the positions for channels (``ch_pos``),
	coordinate frame (``coord_frame``), nasion (``nasion``),
	left preauricular point (``lpa``),
	right preauricular point (``rpa``),
	Head Shape Polhemus (``hsp``), and
	Head Position Indicator(``hpi``).
	E.g.::
	{
	'ch_pos': {'EEG061': [0, 0, 0]},
	'nasion': [0, 0, 1],
	'lpa': [0, 1, 0],
	'rpa': [1, 0, 0],
	'hsp': None,
	'hpi': None
	}
	"""
	# get channel positions as dict
	ch_pos = self._get_ch_pos()
	# _get_fid_coords(self.dig)
	# get coordframe and fiducial coordinates
	montage_bunch = _get_data_as_dict_from_dig(self.dig)
	coord_frame = _frame_to_str.get(montage_bunch.coord_frame)
	# return dictionary
	positions = dict(
	ch_pos=ch_pos,
	coord_frame=coord_frame,
	nasion=montage_bunch.nasion,
	lpa=montage_bunch.lpa,
	rpa=montage_bunch.rpa,
	hsp=montage_bunch.hsp,
	hpi=montage_bunch.hpi,
	)
	return positions

[sappelhoff]

test failure seems to be due to timeout:

=========================== short test summary info ============================
FAILED mne/commands/tests/test_commands.py::test_watershed_bem - Failed: Time...
FAILED mne/commands/tests/test_commands.py::test_flash_bem - Failed: Timeout ...
=============== 2 failed, 3059 deselected in 172

https://dev.azure.com/mne-tools/mne-python/_build/results?buildId=11095&view=logs&j=25a52bd8-0e32-5080-16fd-ef6520ac93ab&t=ed0c6a43-6c02-5c37-64bf-ac1c672f0690

[drammock]

just 2 suggestions, neither are blockers.

[drammock]

thanks @sappelhoff !