Fix Python3 compatibility with the CHARMM parsers.

wrappers/python/simtk/openmm/app/internal/charmm/topologyobjects.py

@@ -12,7 +12,7 @@ Copyright (c) 2014 the Authors
 
 Author: Jason M. Swails
 Contributors:
-Date: July 3, 2014
+Date: August 15, 2014
 
 Permission is hereby granted, free of charge, to any person obtaining a
 copy of this software and associated documentation files (the "Software"),
@@ -347,6 +347,15 @@ class Atom(object):
             retstr += '; %s> ' % (self.name)
         return retstr
 
+    def __lt__(self, other):
+        return self.idx < other.idx
+    def __gt__(self, other):
+        return self.idx > other.idx
+    def __le__(self, other):
+        return not self > other
+    def __ge__(self, other):
+        return not self < other
+
 # ++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
 
 class AtomList(TrackedList):