Fixed Python API for CustomNonbondedForce.addInteractionGroup()

wrappers/python/src/swig_doxygen/OpenMM.i

@@ -20,6 +20,7 @@ See https://simtk.org/home/pyopenmm for details"
 
 %include "std_map.i"
 %include "std_pair.i"
+%include "std_set.i"
 %include "std_vector.i"
 namespace std {
   %template(pairii) pair<int,int>;
@@ -32,6 +33,7 @@ namespace std {
   %template(mapstringstring) map<string,string>;
   %template(mapstringdouble) map<string,double>;
   %template(mapii) map<int,int>;
+  %template(seti) set<int>;
 };
 
 %include "windows.i"