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(...)

src/OpenFOAM/algorithms/dynamicIndexedOctree/dynamicIndexedOctree.H

@@ -452,8 +452,7 @@ public:
             {
                 if (nodes_.empty())
                 {
-                    FatalErrorInFunction
-                        << "Tree is empty" << abort(FatalError);
+                    return treeBoundBox::null();
                 }
                 return nodes_[0].bb_;
             }