Omegascan bug fixes and improvements

The omegascans have overlapping labels, and sometimes the color scheme gets changed (maybe by a parallel matplotlib process?). The color scheme issue can probably be fixed by specifying it in https://git.ligo.org/emfollow/gwcelery/-/blob/main/gwcelery/tasks/detchar.py#L144-148.

It also makes sense to move the omegascan times to not be centered and instead to include more of the time before the merger, since we typically don't care as much about the scan post-merger.