FEAT, API: Read (and store) additional "sample fields" from EDF file

[scott-huberty]

EyeLinkIO currently reads x-position, y-position (using a gaze to pixel-on-screen coordinate frame), and pupil size data from the EDF file. There are other continuous sample fields that can be read from EDF files (continuous meaning, metrics that are reported on a per-sample-basis):

• Eye movement velocity data
• "Head Referenced Gaze Data". This is similar to xpos / ypos, except that the coordinate frame is the subjects ahead (as opposed to the pixel coordinate on the screen). I think a use case for this would be a co-registered EEG Eyetracking experiment, where the experimenter wants to study the relation between eye movement and EOG signals.
• Head distance, xpos, ypos (for EyeLink Trackers that support "Remote Mode").

I don't think that it should be too hard to read this data, but we probably want to include an API for users to specify what "sample fields" they actually want to read from the EDF file.