Track chopper cutouts corresponding to subframes in tof.chopper_cascade?

[SimonHeybrock]

It become apparent in #472 that this will likely be required, since not all chopper cutouts are in use at any given time. To compute the time-of-flight origin we need to be able to tell, given a frame or subframe defined by and computed with tof.chopper_cascade, which openings of a given chopper gave rise to that subframe.

It is not 100% clear if this is the correct approach. Maybe there is a better way of computing or setting the TOF origin?