Clean an enhance hypertree grids.

Fix typos, style & a potential memory leaks.
Add initial support for extent on level 0 rectilinear grid.

Change-Id: I03ddd5a0b5a30901054a59ebe2bee2be38fa2bf8
Merge topic 'CleanHyperTreeGrid' into master

d0c862b4 Clean an enhance hypertree grids.