[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
[robots/grasper] modified grasp service for grasper robot.

The service request grasp_ was modified into grasp(True, "object_name")
in grasper.
It allow a robot to grasp a defined object and attach it to the arm.
If no name is specified it will grasp the 1st object detected by the
Near sensor.
If True is given as 1st parameter, it will grasp the object.
If False, it will release it from robot's hand.

Also the RobotGrasper class has been renamed into GraspingRobot.

Change grasp service parameter.
Fixed armatures and human avatar

Fixed the creation of the armature actuator for the human avatar.
Closes #479

While here:
- make the creation of IK targets more robust to people tweaking settings
  from within Blender
- fixed the 'multiple_humans.py' test which was severly outdated
[builder] Allow control of force/torque through socket and yarp

I don't see any reason why not able it, as it works perfectly