Define, document, and assert/test "standard model output format"

[afmagee42]

          Do you have any validation methods, so that you can check up front that these df's have the schema you expect?

might not be necessary, but it's a few lines of code that can save a lot of headache later

Originally posted by @swo in #51 (comment)

[afmagee42]

I'd propose that we make a

class MultinomialModelOutput(pl.DataFrame):
    def __init__(self, *args, **kwargs):
        super().__init__(*args, **kwargs)
        self.validate()

    def validate(self):
        # Do validation here

and then we can

1. Type-hint returns/inputs for convenience
2. Cast things to this as needed to get error checking