chem.xml add useful rdkit descriptors like number of rotatable bonds

• from issue #10 (closed)
• good idea to output information from RDKit like number of rotatable bonds (wrote temporary jiffy for this temp_jiffy_number_rotatable_bonds_jiffy.py
• there are many other descriptors available - see http://www.rdkit.org/Python_Docs/rdkit.Chem.Descriptors-module.html