ENH: use Zero when zero-initializing types

- makes the intent clearer and avoids the need for additional
  constructor casting. Eg,

      labelList(10, Zero)    vs.  labelList(10, 0)
      scalarField(10, Zero)  vs.  scalarField(10, scalar(0))
      vectorField(10, Zero)  vs.  vectorField(10, vector::zero)

applications/utilities/mesh/conversion/ansysToFoam/ansysToFoam.L

@@ -542,7 +542,7 @@ int main(int argc, char *argv[])
     // Now split the boundary faces into external and internal faces. All
     // faces go into faceZones and external faces go into patches.
     List<faceList> patchFaces(slPatchCells.size());
-    labelList patchNFaces(slPatchCells.size(), 0);
+    labelList patchNFaces(slPatchCells.size(), Zero);
     forAll(boundary, patchi)
     {
         const faceList& bFaces = boundary[patchi];

applications/utilities/mesh/conversion/fluentMeshToFoam/fluentMeshToFoam.L

@@ -936,7 +936,7 @@ int main(int argc, char *argv[])
 
     labelListList cellFaces(nCells);
 
-    labelList nFacesInCell(nCells, 0);
+    labelList nFacesInCell(nCells, Zero);
 
     forAll(cellFaces, celli)
     {