[robots/grasping_robots] Initialize properly DraggedObject game property
[builder] Add a method to enable the grasping capacity to a robot

Enable it for human.
For PR2, I don't have any idea how to call it at the morse.robot.pr2
speaks about Hand.Grasp.PR2, which does not exists ...
Should fix #565
[sensors] Add a radar altimeter sensor

This sensor is supposed to measure the distance of an 'aircraft'
relative to the 'ground'.
[doc] Mention the addition of Radar Altimeter sensor