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/test/boundBox/Test-boundBox.C

@@ -34,7 +34,6 @@ Description
 #include "line.H"
 #include "Random.H"
 #include "treeBoundBox.H"
-#include "cellModel.H"
 #include "bitSet.H"
 #include "HashSet.H"
 #include "ListOps.H"
@@ -59,8 +58,7 @@ int main(int argc, char *argv[])
 {
     #include "setRootCase.H"
 
-    Info<<"boundBox faces: " << boundBox::faces << nl
-        <<"hex faces: " << cellModel::ref(cellModel::HEX).modelFaces() << nl
+    Info<<"boundBox faces: " << boundBox::hexFaces() << nl
         <<"tree-bb faces: " << treeBoundBox::faces << nl
         <<"tree-bb edges: " << treeBoundBox::edges << endl;
 
@@ -113,10 +111,8 @@ int main(int argc, char *argv[])
         Info<<"enclose point " << pt << " -> " << bb << endl;
 
         // restart with same points
-        bb = boundBox::invertedBox;
-        bb.add(point(1,1,1));
-        bb.add(point::zero);
-        bb.add(point(0,1.5,0.5));
+        bb.reset(point::zero);
+        bb.add(point(1,1,1), point(0,1.5,0.5));
         bb.add(point(5,2,-2));
 
         Info<<"repeated " << bb << endl;