Reference implementation of AndersenThermostat did not handle massless particles correctly

platforms/reference/src/SimTKReference/ReferenceAndersenThermostat.cpp

@@ -72,6 +72,7 @@ using OpenMM::RealVec;
 
                   for (int j = 0; j < (int) atomGroups[i].size(); j++) {
                       int atom = atomGroups[i][j];
+                      if (atomMasses[atom] != 0) {
                           const RealOpenMM velocityScale = static_cast<RealOpenMM>(sqrt(BOLTZ*temperature/atomMasses[atom]));
                           atomVelocities[atom][0] = velocityScale*SimTKOpenMMUtilities::getNormallyDistributedRandomNumber();
                           atomVelocities[atom][1] = velocityScale*SimTKOpenMMUtilities::getNormallyDistributedRandomNumber();
@@ -79,6 +80,7 @@ using OpenMM::RealVec;
                       }
                   }
               }
+          }
           
       }