RDF module making use of box diagonals

In the MR to thenew SAXS documentation we discuss the conversion of RDFs to SAXS and the limitation of usual RDF implementations to distances not larger than half the smallest cell vector https://gitlab.com/maicos-devel/maicos/-/merge_requests/279#note_1632906699. It would be useful to either implement our own RDF module or extend MDAnalysis' InterRDF class to make use of box diagonals. The main point is how the density normalization is handled once shells larger than half the smallest cell vector.