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/advanced/refinementLevel/refinementLevel.C

@@ -263,7 +263,7 @@ int main(int argc, char *argv[])
             IOobject::NO_READ,
             IOobject::AUTO_WRITE
         ),
-        labelList(mesh.nCells(), 0)
+        labelList(mesh.nCells(), Zero)
     );
 
     if (readLevel)