burca: remove need for custom SnglBurst class

the burst coincidence code relies on Python's standard library bisect routines to identify ranges of triggers coincident with GPS times, and because the bisect routines cannot be given a separate key function (like sort() can) the trigger objects themselves must implement the desired comparison operation. this patch removes that requirement by maintaining a separate list of trigger times and bisecting it. apart from simplifying the code, this achieves a significant performance improvement.