MRG, ENH: Add mixed source estimate support to compute_source_morph

[larsoner]

Closes #7564.
Closes #7007.

@fleurgaudfernau can you see if it works for your use case? For example this code now runs fine:

Starter code

import mne
import os.path as op
import numpy as np

data_path = mne.datasets.testing.data_path()
subject = 'sample'
data_dir = op.join(data_path, 'MEG', subject)
subjects_dir = op.join(data_path, 'subjects')
bem_dir = op.join(subjects_dir, subject, 'bem')

fname_src = op.join(bem_dir, 'sample-oct-4-src.fif')
fname_aseg = op.join(subjects_dir, subject, 'mri', 'aseg.mgz')

labels_vol = ['Left-Cerebellum-Cortex', 'Right-Cerebellum-Cortex']

# create a mixed source space
src = mne.setup_source_space(subject, spacing='oct4',
                             add_dist=False, subjects_dir=subjects_dir)
src += mne.setup_volume_source_space(
    subject, mri=fname_aseg, pos=10.0,
    volume_label=labels_vol, subjects_dir=subjects_dir,
    add_interpolator=True, verbose=True)

src_fs = mne.read_source_spaces(
    op.join(subjects_dir, 'fsaverage', 'bem', 'fsaverage-ico-5-src.fif'))
src_fs += mne.setup_volume_source_space(
    'fsaverage', pos=5., volume_label=labels_vol,
    subjects_dir=subjects_dir, add_interpolator=False, verbose=True)

morph = mne.compute_source_morph(
    src=src, subject_from=subject, subject_to='fsaverage',
    subjects_dir=subjects_dir, niter_affine=[1, 1, 1],
    niter_sdr=[1, 1, 1], src_to=src_fs, verbose=True)

vertices = [s['vertno'] for s in src]
n_vertices = sum(len(v) for v in vertices)
data = np.ones((n_vertices, 1))
stc = mne.MixedSourceEstimate(data, vertices, 0, 1, 'sample')
stc_fs = morph.apply(stc)

[codecov]

Codecov Report

Merging #7734 into master will decrease coverage by 0.13%.
The diff coverage is 99.42%.

@@            Coverage Diff             @@
##           master    #7734      +/-   ##
==========================================
- Coverage   90.36%   90.23%   -0.14%     
==========================================
  Files         459      455       -4     
  Lines       84942    84750     -192     
  Branches    13459    13429      -30     
==========================================
- Hits        76762    76477     -285     
- Misses       5321     5405      +84     
- Partials     2859     2868       +9     

[larsoner]

Got a report from the user that this works so I'll merge once CIs are happy

[agramfort]

thx heaps @larsoner !