Rename Wheel extractor var names

[k1o0]

The var names are ambiguous in the Wheel extractor as outputs parts of trials, wheel, and wheelMoves objects. This may have lead to #853
https://github.com/int-brain-lab/ibllib/blob/develop/ibllib/io/extractors/training_wheel.py#L411-L412

Suggested changes:

var_names = ('wheel_timestamps', 'wheel_position', 'wheelMoves_intervals',
             'wheelMoves_peakAmplitude', 'wheelMoves_peakVelocity_times', 'trials_firstMovement_times',
             'trials_is_final_movement')