The function mne.add_source_space_distances calculates the distances
between sources on the same hemisphere, but is there a way to get the
distances between sources on different hemispheres as well?
Those source space distances are along the cortical surface as defined by
Freesurfer. Since left and right hemis are separate surfaces, you can't use
that algorithm to get distances between hemispheres. You can instead use a
simple Euclidean distance instead, e.g.
`np.linalg.norm(src[0]['rr'][vertno_idx_left] -
src[1]['rr'][vertno_idx_right]`.