MGridGenGAMGAgglomerate: Use sum magSf rather than mag(sum Sf) for face-weighting

This is more consistent with the faceArea agglomeration and generates more reliable agglomerations.

src/fvAgglomerationMethods/MGridGenGamgAgglomeration/MGridGenGAMGAgglomerate.C

@@ -30,11 +30,6 @@ Description
 #include "fvMesh.H"
 #include "syncTools.H"
 
-//extern "C"
-//{
-//#   include "mgridgen.h"
-//}
-
 // * * * * * * * * * * * * * Private Member Functions  * * * * * * * * * * * //
 
 void Foam::MGridGenGAMGAgglomeration::

src/fvAgglomerationMethods/MGridGenGamgAgglomeration/MGridGenGAMGAgglomeration.C

@@ -59,7 +59,11 @@ Foam::MGridGenGAMGAgglomeration::MGridGenGAMGAgglomeration
 
     // Start geometric agglomeration from the cell volumes and areas of the mesh
     scalarField* VPtr = const_cast<scalarField*>(&fvMesh_.cellVolumes());
-    SubField<vector> Sf(fvMesh_.faceAreas(), fvMesh_.nInternalFaces());
+
+    vectorField magFaceAreas(vector::one*mag(fvMesh_.faceAreas()));
+    SubField<vector> Sf(magFaceAreas, fvMesh_.nInternalFaces());
+    //SubField<vector> Sf(fvMesh_.faceAreas(), fvMesh_.nInternalFaces());
+
     vectorField* SfPtr = const_cast<vectorField*>
     (
         &Sf.operator const vectorField&()