Matlabbatch GUI is not compatible with `pspm_pupil_correct_eyelink`

[dominikbach]

Summary

Matlabbatch provides a channel definition, but pspm_pupil_correct_eyelink (and similarly, pspm_pupil_correct) expects data vectors.

In line with all other PsPM functions, pspm_pupil_correct should act on data channels, not data vectors.

Solution

Include channel loading functionality with pspm_get_channel, including a check on data units (distance, see #887). See pspm_find_valid_fixations for an example.