Protein structure descriptor module.

[cgoliver]

Add protein structure descriptors as structure-level feature sets that can be pre-loaded.

[cgoliver]

Possible descriptors:

• BioZernike --> topological used by PDB
• ProtDCal
• geometricus --> substructure, used by AF analysis paper

[timkucera]

could be provided as custom transforms so users can add what they want, I think we can hardly make a decision on which ones to use here

[pjhartout]

@timkucera @cgoliver would be great to have some tutorials on how to add transforms. I am guessing a similar interface could be used to add additional attributes frequently used like dihedral angles, etc.