Inconsistent handling and documentation of pupil size units

[sappelhoff]

Proposed documentation enhancement

In https://mne.tools/dev/documentation/implementation.html, it says that gaze and pupil data are stored in arbitrary units.

However, in https://mne.tools/dev/auto_tutorials/io/70_reading_eyetracking_data.html, it says:

When possible, MNE-Python will internally convert and store eyetracking data according to an SI unit (for example radians for position data, and meters for pupil size).

When calling raw.get_data(picks=["pupil"], unit="mm"), an error is raised ... however from inspecting my data when getting them without unit, the values DO seem to be in "m" and I can convert to "mm" manually by * 1e3.

EDIT: one important caveat: This is data from a Tobii Pro Spectrum 600 that I imported manually into mne ... and I set the units using mne.preprocessing.eyetracking.set_channel_types_eyetrack