Mypy for recommenders

[Scienfitz]

this is based on PR #146

This fixes types in the recommender package
except:

• assignment errors which cannot currently be solved as we explicitly expect an error if a recommender that cannot handle 0 data is used, this will be fixed once thats enabled for the bayesian one

[AVHopp]

Just minor comments, looks mostly gucci for me. Already approving to avoid blocking, but I'd appreciate if you'd ping me if the other PR is merged such that I can potentially have another look.