Draft: Add scoring function

[p-gw]

closes #9

[p-gw]

I don't particularly like the name scoring_function for the kwarg, but do not have any better idea. Do you have a suggestion @t-alfers?

[t-alfers]

cat_scores, category_scores, cat_scoring,...?

[p-gw]

cat_scores, category_scores, cat_scoring,...?

I think these imply a nominal or ordinal response type.

The function could also be something like x -> x^2 for continuous responses or some weird reverse coding for dichotomous models, x -> 1 - x.