note on gy_vel / gy origin

[behinger]

here we flip gy for eyelink: https://github.com/behinger/etcomp/blob/f30389da49c3416c7a723d44951d197d6e89d40e/code/functions/et_import.py#L314

but we forgot to flip the velocity which is calculated by the eyelink directly. Thus if you calculate the velocity yourself, it will be fliped 90°