Add Radial_Velocity table

[kelle]

We have an ingest_radial_velocity function ready to go. Let's add the table!

[arjunsavel]

@kelle Don't we already have some example quantity tables? My gut feeling is that not everyone would have radial velocities in their own databse.

[kelle]

Hmm, this raises a good topic which I don't think we've discussed explicitly recently -- how do we want to decide what tables to include in the template. At the moment, I'm inclined to include more tables than less...it's easier for a user to remove/modify tables than it is to make up their own from scratch. The more tables we have defined, the more likely databases will be compatible with each other. For example, I think it would make sense for the template to have both a radial velocity AND redshift table.

[kelle]

Partially addressed by #86 . Still will need to add tests from #72