[WIP] Add transparency to plot_alignment

[kdoelling1919]

Reference issue

Issue #8445

What does this implement/fix?

Reduce maximum alpha to be 0.75 in plot_alignment to aid viewing of electrodes inside the brain and skull (mostly sEEG). Also edits a few tutorials to see the new outputs of circleCI

Additional information

The goal currently is to see how these look in the tutorial examples so that we can decide whether this is the best strategy or to use an autodetect (of whether sensors are inside the surface) method instead.

[larsoner]

Looks good so far!

[larsoner]

@kdoelling1919 let me know once you've checked the outputs and they look okay (bonus points if you paste CircleCI links here so that it's easier for me to look, too). Other than one minor gripe, LGTM.

@GuillaumeFavelier what would it take to add a depth_peeling argument to plot_alignment? I think we probably want it for at least some plots. It should probably be another PR, but I suspect it would make at least some of these plots look better.

[kdoelling1919]

Looks good except for now you can see one of the depth electrodes didn't really hit the brain...
https://23107-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/misc/plot_seeg.html#sphx-glr-auto-tutorials-misc-plot-seeg-py

Other tutorials that used plot_alignment seem to be in tact.
https://23107-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/source-modeling/plot_source_alignment.html#sphx-glr-auto-tutorials-source-modeling-plot-source-alignment-py

https://23107-1301584-gh.circle-artifacts.com/0/dev/auto_examples/visualization/plot_eeg_on_scalp.html#sphx-glr-auto-examples-visualization-plot-eeg-on-scalp-py

[agramfort]

this one https://23107-1301584-gh.circle-artifacts.com/0/dev/auto_tutorials/misc/plot_seeg.html#sphx-glr-auto-tutorials-misc-plot-seeg-py looks bad no?

…

[kdoelling1919]

Here's what it looks in current version
https://mne.tools/dev/auto_tutorials/misc/plot_seeg.html#sphx-glr-auto-tutorials-misc-plot-seeg-py

The only difference is that I added the pial surface to show that it can be done with the transparency. It looks fine to me but maybe could be improved by changing the viewing angle?

[agramfort]

all good !

[larsoner]

Suggested change

	- Update ``surfaces`` argument in :func:`mne.viz.plot_alignment` to allow dict for transparency values, and set default for sEEG data to have transparency (by `Keith Doelling`_ (:gh:`8445`))
	- Update ``surfaces`` argument in :func:`mne.viz.plot_alignment` to allow dict for transparency values, and set default for sEEG data to have transparency (by `Keith Doelling`_ (:gh:`8445`)

But we can fix this later, we're probably reformatting all of these lines within this release anyway

[larsoner]

Thanks @kdoelling1919 !

[kdoelling1919]

great success! Thanks to everyone!