ENH: improved zone constructors

- constructor for empty cell/face/point Zones, with contents to be
  transferred in later.

- ZoneMesh::operator(const word&) to return existing zone or a new empty one.

applications/utilities/mesh/generation/extrude/extrudeMesh/extrudeMesh.C

@@ -1026,7 +1026,7 @@ int main(int argc, char *argv[])
             (
                 cutZoneName,
                 frontPatchFaces,
-                boolList(frontPatchFaces.size(), false),
+                false, // none are flipped
                 0,
                 mesh.faceZones()
             )