Fix gripper position

[mikaelarguedas]

It seems that the current position of the gripper frames on Romeo (certainly the same for pepper and nao) are not in the place where the grasping happens but where the motor driving the gripper is.
cf. ros-aldebaran/romeo_robot#10.
The actual offsets from the wrist have been determined empirically by @nlyubova and available here https://github.com/nlyubova/moveit_simple_grasps/tree/romeo-dev/config.
Modifying the urdf generator to add the gripper links using the measured offsets should allow people to grasp object using the l/r_gripper frame as end-effector