Add relationship cardinality constraints

[genedan]

SQLAlchemy can support cardinality constraints in the schema definition. While it would be nice to have these explicitly defined in the code, the way to do so in SQLAlchemy is somewhat not obvious (it is done via an order_by argument to the backref function to indicate one-to-many) - so I'm not sure how much more readable that will make the code unless the reader happens to be very familiar with SQLAlchemy.

Adding this issue here since if any future code needs to have these constraints explicitly defined, we'll need to add them anyway.