Refactor Library API to use OpenFisca's interface?

> The more I think about it, the more I think it would be best to support input/output compatibility with OpenFisca and/or PolicyEngine. Even though PolicyEngine is derived from OpenFisca, they have drifted in some ways from the OpenFisca base (I don't know how much in regard to I/O).
>
> Why?
> - We haven't fully invented an I/O format that will cover all the benefit scenarios we want to support (and OpenFisca/PolicyEngine presumably have). Let's not reinvent the wheel.
> - It would probably make our project more compelling if people could plug it in to one of these other projects which have gained traction. **Our project's reason to exist is to explore how to make it easier to create rules and screeners, not define JSON interfaces.**
>
> Why not?
> - Maybe the OF/PE JSON format is too cumbersome to model in DMN naturally?
> - Maybe the PolicyEngine format is more natural for policy scenario work than individual eligibility screening? I think this is being proven wrong by PE's use in tools like [LA's Benefit Navigator](https://www.loom.com/share/9e012c258a3e4a8e844a91f35fb28988?sid=be377ad8-21c7-40e7-afac-5ca0c32ad88d)
> - Might distract from our main goals in the short term.
>
> I have talked about this idea w/ our friend Nick at Claimant, and he has done some initial experimenting with interoperability with PolicyEngine, but nothing to show for it yet.
>
> This will take some dedication and focus to experiment and come up with a way forward. (It may involve rewriting an extensive amount of our DMN, and/or perhaps supporting "adapters" for converting from one format to another).

_Originally posted by @prestoncabe in [#46](https://github.com/CodeForPhilly/benefit-decision-toolkit/issues/46#issuecomment-2629142069)_

No files, tests, or entry points are named. Start by comparing the current library API with the OpenFisca and PolicyEngine input/output formats, then examine the existing DMN models. Done means agreeing on a compatibility approach, such as direct support or adapters, and defining its scope.

Written by the indexing model from the issue text.