Backported PROPKA code base to Python2.

Now this same code can be run with either Python2.7 or Python3.

Source/group.py

@@ -2,6 +2,7 @@
 # Class for storing groups important for propka calculations
 #
 
+from __future__ import division
 import Source.ligand, Source.determinant, Source.ligand_pka_values, math, Source.protonate
 
 my_protonator = Source.protonate.Protonate(verbose=False)