Bug fix

Was introduced with CamShift (my mistake in suggesting a hack to Carlo).

In fact, because of the way stopwatches are implemented, it is not
possible to call plumed.cmd recursively. I made a quick fix (the
right fix would be to adjust stopwatches)

src/core/GREX.cpp

@@ -65,7 +65,7 @@ void GREX::cmd(const string&key,void*val){
   }else if(key=="setMPIIntercomm"){
     CHECK_NOTINIT(initialized,key);
     intercomm.Set_comm(val);
-    plumedMain.cmd("setMPImultiSimComm",&intercomm.Get_comm());
+    plumedMain.multi_sim_comm.Set_comm(val);
   }else if(key=="init"){
     CHECK_NOTINIT(initialized,key);
     initialized=true;