Update t_0 for superevent to be the time of the new preferred event

When the preferred event of a superevent is updated, the start and end times of the superevent time interval may be extended (depending on the exact timing), but we have observed that the t_0 never changes -- it stays equal to the time of the original event which initiated the superevent. This has confused people who expected that the t_0 of the superevent should represent the time of the preferred event.

The gracedb.update_superevent routine does take t_0 as a kwarg, so this issue can be addressed by a change to superevents.py in gwcelery. I suggest doing it within _update_superevent, since the preferred event is known within that routine and it should be straightforward to grab the time of that preferred event and add a 't_0' kwarg to be passed along in the call to gracedb.update_superevent .