Use SI units for eyetracking data, update tutorials

[scott-huberty]

closes #12756

• Updated mne.defaults so that eyegaze channels are expected to be radians and pupil size channels are expected to be meters. When specifying the scalings, I matched the scalings with other MNE channel types that have a radians/meters unit (e.g. fnirs_fd_phase for radians, hbo/hbr for meters).

• Updated the eyetracking tutorials such that eygaze units are converted to radians as early as possible.

Notable exceptions:

• I did not update the example plot_eyetracking_heatmaps. I'd have to play with the code a bit to make the heatmap overlay with the stimulus image (which is in pixels) nicely. Maybe a follow up PR?
• in this PR I did not demonstrate converting Eyelink Pupil size data to meters. As I've mentioned before I don't think this is trivial, and I have never done it with my Eyelink data. I might need to do it for an analysis in the coming months and so I would prefer to address this when I cross that bridge.

[scott-huberty]

Ok here is the built documentation for the Eyetracking tutorials. Marking as ready for review!

• Working with eyetracking data in MNE-Python
• Importing data from EyeLink devices
• Plotting Eyetracking heat maps in MNE-Python

[larsoner]

This ends up a bit wacky:

If you set the units and scalings it will probably be more reasonable I think

[scott-huberty]

Sorry @larsoner Im not sure what you mean by setting the units and if you are referring to the eyegaze channel or pupil channel (I assume eyegaze) - Can you clarify?

[larsoner]

I meant for plot_raw but I see now that we only have it for other viz functions like plot_evoked, so never mind! Also not sure it would actually help...

[larsoner]

Thanks @scott-huberty !

[drammock]

looking a bit late, but why is the unit for pupil µM instead of µm?? M is moles / molar

[larsoner]

Ugh yes that seems like a bug...