Add an interface to use `extra-model` inside of Python

[Misha Balyasin]

Right now all of the interfaces we expose assume the workflow like this: csv file in -> csv file out.
It would be great to expose another interface that takes pandas df and returns pandas df as an output. We do this internally anyways, so nothing super difficult.

[Rushi Chaudhari]

Hi can anyone review this PR for this issue #198, thanks

[Misha Balyasin]

Solved with #199