Force field optimization fix

[erm42]

Taking advantage of the more robust RDKit mol object builder from PR #36 , I clean up the generation of INCHI keys (yarp/yarpecule/yarpecule.py -> self.get_inchi()) and attempt to make a quick fix for issue #31

Overview of changes:

• Generate INCHI keys via RDKit mol object. This avoids the need to write a temporary MOL file to disk. Turns out, also, you don't need the geometry in order to generate the INCHI key, at least not the first 14 characters, which is all that we care about
• Quick bug fix in the helper/read_pkl.py script (pretty sure I broke this in PR Pre-enum filter: product blinders #29 )
• Added FF opt functions for both RDKit and Open Babel in yarp/util/ folder
• Updated the quick_geom_opt() function in yarp/reaction/generate_rxns.py to first attempt to form the product geometry using RDKit, then fall back to Open Babel if RDKit changes the connectivity of the product. If neither RDKit nor Open Babel can form a valid product with UFF, we throw the reaction out (verbose messages should trigger)
• Added testing cases for 3 molecules, two of which fail for RDKit/pass for Open Babel FF optimizations, and one which fails for Open Babel/passes for RDKit.