trying to improve point on boundary check (sketch)

- it seems I have dropped one difference vector whilst I thought I'd need five
  of those.
- figured out the signs they should be correct now.
- simplified known_point===undefined branch
- tests awful
- tested awful ok because I fucked up _list/bbox-filter
- and because the bbox pre-filter was too strict
- utils.js makes sense now but _list/bbox-filter is way fucked up
- this looks about right