ENH: creating a bounding box without points yields an inverted box

- The code create a box with a (0,0,0) point.
  The new definition is more logical and makes it very easy to grow
  the bounding box to include new points. It also simplifies much of
  the logic in the constructors.

- Use ROOTVGREAT instead of VGREAT for sizing greatBox and invertedBox.
  Avoids some overflow issues reported by Mattijs (thus GREAT has been
  used in treeBoundBox), but might still need further revision.

src/sampling/sampledSet/patchCloud/patchCloudSet.C

@@ -74,7 +74,7 @@ void Foam::patchCloudSet::calcSamples
 
     labelList patchFaces(sz);
     sz = 0;
-    treeBoundBox bb(treeBoundBox::invertedBox);
+    treeBoundBox bb(boundBox::invertedBox);
     forAllConstIter(labelHashSet, patchSet_, iter)
     {
         const polyPatch& pp = mesh().boundaryMesh()[iter.key()];