Motion filter revision

A minor version update, which makes a change to the default implementation of the bandstop motion filter.

For context, the default filter behavior to date has been to apply a second-order Butterworth filter bidirectionally with filtfilt() twice, meaning that when the default motion_filter_order = 4 was used, the filter was actually 8th-order.

This revision halves the number of motion filter repetitions when using filtfilt(), but makes no change to the unidirectional filter options.

(In testing the revised default filter on resting-state data of a sample of 51 ABCD participants, using filter frequency band of [18.582, 25.726] bpm, post-filtering mean FD changed from 0.1875 to 0.2085 mm, and proportion of frames with FD < 0.2 changed from 75.58% to 72.33%.)

Credit to user @cindyhfls for raising awareness of this issue.