Create a first implementation for Recall@k

[mam10eks]

Dear all, as mentioned in the issues https://github.com/joaopalotti/trectools/issues/17 and https://github.com/joaopalotti/trectools/issues/40, there is currently no implementation of Recall@k. This pull request creates a first implementation (not very efficient, as I used iterrows to keep it simple, but we could later improve upon that :)).

I mostly copied the code from other methods.

Best regards, Maik

[hscells]

It looks good from my side. LGTM?