- 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.
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