move SQLAlchemy functions from ocf dashboard to here

[rachel-labri-tipton]

Detailed Description

There are several SQLAlchemy functions in the OCF Dashboard that it would make more sense to have in the pv-site-datamodel repo.

These functions can be found here in the get_data.py file in the OCF Dashboard code.

Context

These functions were not originally in the datamodel so were written for the dashboard functionality, but they interact directly with the database, and it would make sense for them to be accessible via the datamodel.

Possible Implementation

• Add the functions and tests to the pv-site-datamodel.
• Change the imports in the OCF Dashboard code.