snappyHexMesh: Changed the default for the writing of pointLevels and cellLevels files in the polyMesh directory

These files are now only written if the "refinement" write flag is set:

writeFlags
(
    .
    .
    .
    refinement
);

tutorials/mesh/snappyHexMesh/flange/system/snappyHexMeshDict

@@ -301,6 +301,7 @@ writeFlags
     scalarLevels    // write volScalarField with cellLevel for postprocessing
     layerSets       // write cellSets, faceSets of faces in layer
     layerFields     // write volScalarField for layer coverage
+    refinement
 );