Remove option to provide gaze data in pixels in `pspm_pupil_correct_eyelink`

[dominikbach]

Summary

pspm_pupil_correct_eyelink includes an option to provide gaze data in pixels. This calls a subfunction which contains a bug (screen resolution is user-provided and user is likely to provide the display screen resolution, whereas what is needed is the resolution of the eyetracker screen, which is already stored with the gaze data). Also, this duplicates the function pspm_gaze_convert

Solution

• remove option to provide gaze data channel in pixels
• add check on the units of the provided gaze data

see also #888