Add spherical montages

[cbrnr]

Fixes #7141. I used the 3D location files from here.

[sappelhoff]

LGTM! Thanks for the effort @cbrnr

I do think we should insert references about where the colin and spherical locations come from (fieldtrip/robert and eeg_positions, respectively). Just so that we have a clear documentation of the provenance.

As for where, I am open.

[larsoner]

I do think we should insert references about where the colin and spherical locations come from (fieldtrip/robert and eeg_positions, respectively). Just so that we have a clear documentation of the provenance.

How about a new mne/channels/data/montages/provenance.rst (or .md or .txt)? Can keep it as dev-level info for now I think and we just want some way to document it clearly. We can start this new file with just info for the stuff added here, but @cbrnr if you want to dig into the history of the other files and add it here that's great too.

Maybe someday in the future we can add to some docstring or something if needed, by I'd assume YAGNI / not enough benefit to doing it for now

@cbrnr next time you push a commit can you put [circle full] in the commit message? Should help ensure no examples break. (CircleCI will already run the examples you modified here, but the commit-message trick will help ensure errors in other, non-modified examples aren't missed.)

[cbrnr]

@larsoner all done!

[larsoner]

Just one tweak, will push the suggested commit with [circle full] and mark for merge-when-green, thanks in advance @cbrnr !