Schema - Refine relationship declarations

[jomey]

As noted by @aaarendt in this comment, we should revisit all of our relationship definitions and add the back_populates option.

Docs here:
https://docs.sqlalchemy.org/en/20/orm/basic_relationships.html#one-to-many

[aaarendt]

Encountered this error while working on SnowEx/snowex_db#65; putting it here for documentation:

SAWarning: relationship 'Site.layer_data' will copy column sites.id to column layers.site_id, which conflicts with relationship(s): 'LayerData.site' (copies sites.id to layers.site_id). If this is not the intention, consider if these relationships should be linked with back_populates, or if viewonly=True should be applied to one or more if they are read-only. For the less common case that foreign key constraints are partially overlapping, the orm.foreign() annotation can be used to isolate the columns that should be written towards.   To silence this warning, add the parameter 'overlaps="site"' to the 'Site.layer_data' relationship. (Background on this warning at: https://sqlalche.me/e/20/qzyx) (This warning originated from the `configure_mappers()` process, which was invoked automatically in response to a user-initiated operation.)
  obj = session.query(clz).filter_by(**check_kwargs).first()