made by user.

- Add timestamp to object AbstractPoint

- Add dependencies tu use manage.py dbshell

feat: bouttons avec le slider suivant #79

See merge request !1

It is no longer possible to instantiate an abstract model, so we have to
hack around that.

It uses the API to estimate the ground altitude for each point, and then
sets the remaining altitude to "height above ground".  The total altitude
(ground altitude + height above ground) remains unchanged.

To see what would happen with your data without saving the result, run:

    ./manage.py fix_ground_altitude --dry-run

If you're happy with the result, run it for real:

    ./manage.py fix_ground_altitude

Two altitude providers are now used by default:

- a very accurate provider, but limited to France
- as a fallback, a less accurate provider, but with a global coverage