Formalize schema of deserialized output with Pydantic

[JohnGiorgi]

Right now, we have a deserialized output format that looks like:

[
    {
        "ADE": [
            (("fenoprofen", "DRUG"), ("pure red cell aplasia", "EFFECT"))
        ]
    }
]

it would be nice to capture this schema with a Pydantic model. I think this would simplify some of the code that has to interact with this object, but it would also make it easier to save/load this from disk.