Add support for stop_motion service

[gavanderhoorn]

As mandated by the Industrial Robot Driver Specification, section 1.3.3, under Services:

• stop_motion (industrial_msgs/StopMotion)
  • stop current robot motion
  • can resume motion by sending a new motion command

[JeremyZoss]

This should already be supported by the core industrial_robot_client libraries. See here and here.

Currently, it always returns TRUE. This could be enhanced, but we would want to clarify whether true/false status reflects whether the robot received the message (e.g. ACK) or whether the motion was actually stopped (SUCCESS). The latter would likely require custom support on each manufacturer's driver.

[gavanderhoorn]

As in #11: as fanuc_driver subclasses the industrial_robot_client it automatically supports this.

As to the comment on the current implementation, see ros-industrial/industrial_core#26.