FIX: fiducials locking in coreg gui

[GuillaumeFavelier]

This PR fixes the issue with CoregistrationUI reported in #8833 about the fiducial markers being unlocked when dig info is provided.

It's an item of #8833

cc @larsoner

[GuillaumeFavelier]

Does it fix your issue @larsoner ?

[larsoner]

It's at least progress in the right direction in that it's not in a mixed state. Now it starts in a "coregistering" state.

The way the old version worked was different (and better IMO). If there was no -fiducials.fif, it would start in a "setting fiducials" state, and there was a "save" button. In other words it looks like this on first load:

MNE_3D_BACKEND=mayavi mne coreg -s sample -d ./subjects --fif MEG/sample/sample_audvis_raw.fif --trans MEG/sample/sample_audvis_raw-trans.fif

Moreover, if I'm happy with the MNI-based fiducials -- or if I do some click-based adjustments -- I can then just click the "save" button, and it saves it to the correct path, and locks the fids:

Then on a close-reopen, because SUBJECT/bem/SUBJECT-fiducials.fif now exists, it will just load directly into this "coregistering" state.

This is useful because the MNI-based fiducials are only a suggestion / starting point. So when -fiducials.fif is missing, pushing people into a "setting fiducials" state (that they can quickly disable permanently by saving or just for this session by locking) gets them to think about/set the fiducials properly, which can really help with coreg, especially in matched mode.

Do you want to try implementing that workflow here? If not, we can add it to the TODO. To me it would be the next highest priority for 0.24 but not a blocker for release.

[larsoner]

Thanks @GuillaumeFavelier !