Allow multiple clips with same detector, recording channel, and start time.

[HaroldMills]

Vesper currently does not allow more than one clip with the same detector, recording channel, and start time in an archive. This is accomplished via a multicolumn uniqueness constraint on the clip table. I included the constraint to prevent duplicate clips from being created by accidentally running a particular detector on a particular recording more than once. The problem with that is that there are legitimate reasons for creating more than one clip for a given processor, recording channel, and start time. As one example, an upcoming detector sometimes creates multiple clips with the same start time for overlapping detections at multiple taxonomic levels. So I think it would be best to remove the constraint and rely on another means of dealing with undesired duplicate clips. Right now it's possible to delete a job from the database, which also deletes the job's clips, but that is not a good solution since we'd like to maintain a complete record of the jobs that have been run. A better solution might be a command that deletes clips created by a particular job, perhaps only for certain sensors. It would also be desirable to add a command that can detect duplicate clips in a database, regardless of which job created them, and optionally delete them.