ev3cv controller demo

Demo comparing using ev3dev::motor with ev3cv::servo when continuously updating the position setpoint. The demo uses two motors, one used as a controller, the other as a follower. The follower motor attempts to match position of the controller with minimal latency.
For the ev3dev::motor demo, I used the best settings I could find while keeping latency low. Using the ramp setting helps (marginally, there is still significant oscillation), but increases latency/sluggishness.