Implement events and superevents as segments for easy segment operations

Right now the the superevent clustering checks if an event gpstime falls inside a superevent window. This can be made more robust and dynamic by using the glue.segments library and implementing both events and superevents as segments. The superevent list pulled down from GraceDb could be implemented as a segmentlist for easy operations like intersection, shifting etc.

• Implement event and superevent as segment/segmentlist.
• Use segments library to perform clustering operations.