Superevent queries

This issue is created to get some feedback on what attributes would be most useful for querying for superevents, as well as which attributes should be added to event queries. Here's what I came up with off the top of my head.

Superevents

• Search by superevent ID (date-based)
• Superevent submitter
• Ranges of t_start, t_0, t_end (GPS times)
• Whether an alert has been sent out for the superevent
• Whether the superevent is confirmed as a GW
• Preferred event graceid (possibly a range of graceids)
• Event graceids
• Labels
• Do we need the ability to find superevents based on attributes of their preferred events? Ex: find all superevents with a preferred event which was identified by the CWB pipeline. If this seems useful, which attributes?

Events

• Find all events which are preferred events for superevents
• Find all events which are part of a particular superevent
• Find all events which are part of any superevent (or are NOT part of any superevent)

If there are other filters which would be useful, or if any of these don't seem useful, please comment!