Import REFERENCE_TABLES from SIMPLE-db

[kelle]

Right now, the reference tables are hardcoded in
utils.py. Instead, we should figure out a way to import simple and get the latest reference tables from there.

Right now they are in simple/init.py but I'm thinking we should have a dedicated function:
SIMPLE-AstroDB/SIMPLE-db#627. But it looks like the web app just needs the reference_tables list. I'll add that to the spec for 627.

[dr-rodriguez]

It might be possible to do this as simply as from simple import REFERENCE_TABLES
where simple is in the requirements.txt as:

simple @ git+https://github.com/SIMPLE-AstroDB/SIMPLE-db