Undid more erroneous changes.

platforms/reference/src/ReferenceKernels.cpp

@@ -1004,7 +1004,6 @@ void ReferenceCalcCustomNonbondedForceKernel::initialize(const System& system, c
     
     for (int i = 0; i < force.getNumInteractionGroups(); i++) {
         set<int> set1, set2;
-	printf("initializing interaction group %d\n", i); // DEBUG
         force.getInteractionGroupParameters(i, set1, set2);
         interactionGroups.push_back(make_pair(set1, set2));
     }

platforms/reference/src/SimTKReference/ReferenceCustomNonbondedIxn.cpp

@@ -177,6 +177,7 @@ void ReferenceCustomNonbondedIxn::calculatePairIxn( int numberOfAtoms, vector<Re
     }
     if (interactionGroups.size() > 0) {
         // The user has specified interaction groups, so compute only the requested interactions.
+        
         for (int group = 0; group < (int) interactionGroups.size(); group++) {
             const set<int>& set1 = interactionGroups[group].first;
             const set<int>& set2 = interactionGroups[group].second;