Number exact matching and proximity search

This is to address https://github.com/geoadmin/mf-chsdi3/issues/628

- Integers are treated separately
- I replaced the word by word matching (which was ranked to strongly before) with a proximity match
- Bug in chsdi/lib/validation.py has been fixed

Ranking is really improved according to me. All the intergration tests passed, so it means that the rules we created are respected.

In general, less results are returned and words order is now more important.

This needs intense testing...