Fix for METAD multiple walkers

When using METAD with file-based multiple walkers and many processes
launched with mpirun (e.g. with gromacs -multi) replica suffix is
not added to grid file.

(see also 667da1b3)

Contributed by Marco De La Pierre

src/bias/MetaD.cpp

@@ -875,6 +875,7 @@ last_step_warn_grid(0)
       if(r>0) gridfilename_="/dev/null";
       gridfile_.enforceSuffix("");
     }
+    if(mw_n_>1) gridfile_.enforceSuffix("");
     gridfile_.open(gridfilename_);
   }