protection to set monoatomic when using argon

Author: CatarinaGarbacz

Common/src/CConfig.cpp

@@ -3557,6 +3557,10 @@ void CConfig::SetPostprocessing(unsigned short val_software, unsigned short val_
       SU2_MPI::Error("ARGON is not working with SU2_NONEQ fluid model!", CURRENT_FUNCTION);
   }
 
+  if (GetGasModel() == "ARGON" && GetMonoatomic() == false){
+      SU2_MPI::Error("If you're running an argon mixture, set MONOATOMIC= YES in the .cfg file!", CURRENT_FUNCTION);
+  }
+
   if(GetBoolTurbomachinery()){
     nBlades = new su2double[nZone];
     FreeStreamTurboNormal= new su2double[3];