Store references to the System and Integrator inside the Context so they don't...

Store references to the System and Integrator inside the Context so they don't get garbage collected

wrappers/python/src/swig_doxygen/swig_lib/python/pythoncode.i

@@ -266,3 +266,8 @@ def stripUnits(args):
         newArgList.append(arg)
     return tuple(newArgList)
 %}
+
+%pythonappend OpenMM::Context::Context %{
+    self._system = args[0]
+    self._integrator = args[1]
+%}