[builder] Introduce the notion of GroundRobot

A Ground Robot is basically a robot which can not control its altitude.
[core] If it is a ground robot, do not apply vz command

Fix #459
[builder] Increase radius of morsy robot

I'm not sure what does it change, but it avoids than Morsy go through
the ground ...
[template] Use a GroundRobot as default (to match Morsy semantic)
[actuators] Switch back keyboard ControlType to position

velocity works, but as it is not possible (yet) to have two velocity
controllers, it is often not very useful.
Discussed with pkoch@
[doc] Mention GroundRobot

While here, fix several typos