[ENH] Add adjacency for cortical and volume labels

[raphbrd]

PR description

Fixes #8646

Add two functions in the public API to determine whether labels are adjacent in source spaces:

• mne.label_adjacency(labels, src) for surface spaces.
• mne.volume_label_adjacency(src, subject, subjects_dir, aseg="auto", labels=None) for volumetric spaces.

The typical inputs are any surface parcels and segmentation files from the FreeSurfer recon process (e.g., aparc.a2009s+aseg.mgz)

Additional information

Adjacency between labels is defined as having at least one adjacent voxel, based on the source-space adjacency computed using the existing mne.source_estimate.spatial_src_adjacency function.

For the volumetric case, there is currently no dedicated label object. The current workaround is to use a list of surface labels with hemi='lh' by convention. If that's useful, we could add a VolLabel object in the future.

[larsoner]

Just one tiny efficiency nitpick, otherwise LGTM! Let me know when you push and I'll mark for merge-when-green

[larsoner]

/home/circleci/project/doc/[towncrier-fragments]:194: WARNING: py:func reference target not found: mne.label_adjacency [ref.func]
/home/circleci/project/doc/[towncrier-fragments]:194: WARNING: py:func reference target not found: mne.volume_label_adjacency [ref.func]

[larsoner]

(I'll push a fix)

[welcome]

🎉 Congrats on merging your first pull request! 🥳 Looking forward to seeing more from you in the future! 💪

[wmvanvliet]

Thanks @raphbrd!