Add SourceCatalog

[moeyensj]

We've been discussing consolidating our input type definitions across ADAM-ETL, THOR, the impact project, etc.. This PR adds a de-normalized and flattened input type called SourceCatalog with methods to convert subsets of its columns into the more compute-oriented types.

I think we will also want to add an "Observations" type that stores the relevant subset columns in their appropriate SubTable types. So something like:

class Observations(qv.Table):
    id = qv.LargeStringColumn()
    coordinates = SphericalCoordinates.as_column()
    observers = Observers.as_column()
    ... etc

    def from_source_catalog(cls, source_catalog):
         ...

This is still a WIP.

[akoumjian]

I looked at precovery indexing and this should work fine for it.
Do we want a method to get the healpixel at a certain resolution? Or does that better belong on SphericalCoordinates?

[moeyensj]

I think attaching it to SourceCatalog is totally fine.