Hold and free

[adeguet1]

I propose to add two new commands to CRTK:

• hold, a command to instruct arm to stay in place. Implementation would be robot dependent. For example, dVRK could use servo_jp(setpoint_jp()) while the Galen would use servo_jv(0).
• free, a command to "release" the robot. Often required on dVRK MTM and other haptic devices. Specification would be:
  • Turn on gravity compensation if available
  • Set effort to zero (similar to servo_cf(0) or servo_jf(0))

free could be added to other robots such at the UR or even the Galen (zero force compliance).