MRG, ENH: Add cortex, fix sensors

[larsoner]

Closes #9744

1. Fixes bug introduced in [ENH, MRG] Add mne.viz.Brain.plot_sensors and refactor mne.viz.plot_alignment #9585 (cc @alexrockhill) where fNIRS and projected EEG sensors were not scaled properly based on units

2. Fixes a bug where we iterated over some views twice (in the case of hemi='both') when we should have just done it once -- this is fixed by iterating over 'vol' as the hemi

3. Adds support for custom cortex color mappings in mne.viz.Brain, which is something I've wanted for a while and was necessary to restore this image from stable while still using mne.viz.Brain instead of plot_alignment:

   

4. Streamlines the iteration over views by calling .subplot inside _iter_views rather than doing it manually each time (cc @GuillaumeFavelier )

[larsoner]

@rob-luke render fixed:

[alexrockhill]

Thanks for cleaning this up @larsoner

[rob-luke]

Brilliant, thanks @larsoner very neat