Add demo mode for all weeding implements

[pascalzauberzeug]

#143 introduced a demo mode where the tornado stops just above the ground, which is useful for demonstrations on trade fairs but also for debugging.

We should build this for all weeding implements.
We discussed two points for this:

1. Having the demo height as a setting in the config of the robot Link
2. Save is_demo in the implement persistence, not in the individual tool Link