validate bounding box

I know it should be easy, but in all my demos I seem to have to take like 3 tries at getting the bounding box right 😆

Things I manage to get wrong (so far):
- order obviously
- omitting a negative value

The issue does not name a file, test, or entry point; start by locating where the bounding box is accepted for demos or simulations. Done should mean invalid coordinate order and omitted negative values are detected clearly before processing.

Written by the indexing model from the issue text.