Added barriers

It seems that sometime on travis a regtest with mw-mpi crashes.

I think this is because when using multiple processors per walker
there is a problem in the opening of the old file (for read) and
its backup (for write). Adding this barrier seems to make it more robust

src/bias/MetaD.cpp

@@ -658,6 +658,9 @@ last_step_warn_grid(0)
    }
   }
 
+  comm.Barrier();
+  if(comm.Get_rank()==0) multi_sim_comm.Barrier();
+
 // open hills file for writing
   hillsOfile_.link(*this);
   hillsOfile_.open(ifilesnames[mw_id_]);