User supplied sequence/catalog

[emirkmo]

This is a feature requested by Max. Easiest way to implement seems to be to add a way to define a local sequence by supplementing the api.catalogs call in flows/photometry.py with a local/custom catalog.

The catalog would need ra, dec, epoch, frame, proper motions, and magnitudes in relevant bands. What else? I guess we can generate a unique custom starid for each. It would also need to match the astropy.table formatting of flows' native catalog.