effort / velocity limit placeholders

[iamtesch]

For the "gripper" xacro, there are placeholder limits on the effort and velocity that were necessary to add in order to add position limits. Because the gripper can be controlled by different actuators, and the actuator is a separate component, technically these are undefined, but must be added to the URDF.

We should consider a cleaner solution here, perhaps by adding a param of "driving module type".