Fix classification report for multilabel

[tholor]

This fixes #148.

In multilabel classification we evaluate by predicting multihot vectors (ids), whereas in binary classification we predict label values (strings). Therefore, we need to supply different types of lists to the labels arg of sklearn's classification report: list of ids for multilabel vs. list of strings for binary.

Quickfix: distinguishing these cases

Longterm: We might wanna predict also label values (strings) in multilabel classification. However, we then also need to find/implement another classification report