ENH: general boundBox/treeBoundBox improvements

- null() static method
  * as const reference to the invertedBox with the appropriate casting.

- boundBox inflate(random)
  * refactored from treeBoundBox::extend, but allows in-place modification

- boundBox::hexFaces() instead of boundBox::faces
  * rarely used, but avoids confusion with treeBoundBox::faces
    and reuses hexCell face definitions without code duplication

- boundBox::hexCorners() for corner points corresponding to a hexCell.
  Can also be accessed from a treeBoundBox without ambiguity with
  points(), which could be hex corners (boundBox) or octant corners
  (treeBoundBox)

- boundBox::add with pairs of points
  * convenient (for example) when adding edges or a 'box' that has
    been extracted from a primitive mesh shape.

- declare boundBox nPoints(), nFaces(), nEdges() as per hexCell

ENH: return invertedBox instead of FatalError for empty trees

- similar to #2612

ENH: cellShape(HEX, ...) + boundBox hexCorners for block meshes

STYLE: cellModel::ref(...) instead of de-reference cellModel::ptr(...)

applications/utilities/preProcessing/createBoxTurb/createBlockMesh.H

@@ -1,5 +1,3 @@
-const cellModel& hex = cellModel::ref(cellModel::HEX);
-
 cellShapeList cellShapes;
 faceListList boundary;
 pointField points;
@@ -8,20 +6,8 @@ pointField points;
 
     block b
     (
-        cellShape(hex, identity(8)),
-        pointField
-        (
-            {
-                point(0, 0, 0),
-                point(L.x(), 0, 0),
-                point(L.x(), L.y(), 0),
-                point(0, L.y(), 0),
-                point(0, 0, L.z()),
-                point(L.x(), 0, L.z()),
-                point(L.x(), L.y(), L.z()),
-                point(0, L.y(), L.z())
-            }
-        ),
+        cellShape(cellModel::HEX, identity(8)),
+        pointField(boundBox(point::zero, L).hexCorners()),
         blockEdgeList(),
         blockFaceList(),
         N