ENH: Add trigger-time-file and trigger-time-tuple to allow sequences of trigger times

Adds two ways to set a sequency of trigger times, analogous to gps_file and gps_tuple respectively.

This is one potential way to handle sequences of trigger times. The other would be to interpret from the gpstimes what the sequence of trigger times should be, assuming the same set of offsets. This route is more straightforward and flexible, but requires additional inputs in order to achieve the same goal.