support drop_channels method in DigMontage

[sappelhoff]

I would like to add a remove_channels method to

https://mne.tools/dev/generated/mne.channels.DigMontage.html#mne.channels.DigMontage

Could this be as simple as adjusting the remove_fiducials method?

mne-python/mne/channels/montage.py

Lines 677 to 702 in ace9381

	@verbose
	def remove_fiducials(self, verbose=None):
	"""Remove the fiducial points from a montage.
	Parameters
	----------
	%(verbose)s
	Returns
	-------
	inst : instance of DigMontage
	The instance, modified in-place.
	Notes
	-----
	MNE will transform a montage to the internal "head" coordinate
	frame if the fiducials are present. Under most circumstances, this
	is ideal as it standardizes the coordinate frame for things like
	plotting. However, in some circumstances, such as saving a ``raw``
	with intracranial data to BIDS format, the coordinate frame
	should not be changed by removing fiducials.
	"""
	for d in self.dig.copy():
	if d["kind"] == FIFF.FIFFV_POINT_CARDINAL:
	self.dig.remove(d)
	return self

The reason I need this is that I am working w ith an experimental montage, which I obtain by "tuning" an existing montage (renaming, dropping).

[larsoner]

I think the public API should probably be remove_channels to match that of our other classes (e.g., Raw). But what you do under the hood could be refactoring remove_fiducials to be _remove_dig_points(...) and then make the public remove_fiducials and remove_channels both make use of _remove_dig_points for example