Improve Python 2 compatability with "future" print_function.

Use python3 version of the print function when running
under python2. Also added "from __future__ import division"
to a few more module files.

Source/group.py

@@ -3,6 +3,8 @@
 #
 
 from __future__ import division
+from __future__ import print_function
+
 import Source.ligand, Source.determinant, Source.ligand_pka_values, math, Source.protonate
 
 my_protonator = Source.protonate.Protonate(verbose=False)