scATAC: Annotation of features from adata and gtf

Note: not related to unit testing

This is a merge request for the annotation of features within the adata.var table for scATAC-seq. The function is 'sctoolbox.annotation.annotate_features'. It uses UROPA in the background, and adds gene annotations as additional columns in .var.

I would be happy if you would try it out and provide feedback 🙏