Generalize functions to include Scikit fingerprints support

[florian-huber]

Changed the fingerprint computation code quite a bit.
It now can run with rdkit as well as scikit-fingerprints type generators.

Some inconsistency between both packages is the use of the term sparse, which means unfolded fingerprints in rdkit, but scipy.csr formated versions of folded fingerpritns for scikit-fingerprints.

Here, I now switched to a general distinction of three categories:

• count: True / False --> either counting occurences or simple binary presence/absence
• return_csr: True / False --> when True will return the folded fingerprints in csr format
• folded: True / False --> when False, will not output fixed-sized fingerprints but arrays of integer bits