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.