First generate catalog for one tract or patch

[jiwoncpark]

So far, we've been using the minion_1016 opsim database for a single position. We want to eventually use the full, position-dependent opsim db, so we restrict ourselves to a small area of the sky (given tract or patch) for quick development. Later, we can iterate over the tracts or patches. We also want to add stars from the truth catalog in this chosen tract or patch.