SQUASH: simplify the weighted average routine

src/functionObjects/field/fieldStatistics/fieldStatisticsImpl.cxx

@@ -121,8 +121,7 @@ T Foam::functionObjects::fieldStatistics::calcMean(const Field<T>& field)
 {
     if (internal_ && (mean_ == VOLUMETRIC))
     {
-        const Field<scalar>& V = mesh_.V();
-        return (gSum(V*field)/gSum(V));
+        return gWeightedAverage(mesh_.V(), field);
     }
 
     return gAverage(field);