Merge pull request #1080 from peastman/exec

Fixed errors executing force field scripts on Python 3

wrappers/python/simtk/openmm/app/forcefield.py

@@ -532,7 +532,7 @@ class ForceField(object):
         # Execute scripts found in the XML files.
 
         for script in self._scripts:
-            exec script
+            exec(script, locals())
         return sys