Save more space in zbox by omitting cached bounding_box
Save more space in zbox by omitting cached bounding_box

This doesn't affect speed (maybe it's a bit slower) but
not-wasting-lots-of-memory is still a great goal.

We could more memory-efficiently cache the bbox in a way
where coords_ *is* bbox_.min (possibly at the cost of code
clarity), but it would still waste 24 bytes for a
64bit x 3dim setup, not the previous 48 bytes.

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