Dynamixel trajectories

[hello-binit]

This PR makes the trajectory interface available for the Dynamixel motors.

• follow_trajectory(): initiates execution of a waypoint trajectory
• update_trajectory(): used to update the hardware with the next segment of the waypoint trajectory (rarely used by users)
• stop_trajectory(): used to stop execution of a waypoint trajectory

This PR includes unit tests of the new methods, and a small example program is reproduced here:

import stretch_body.stretch_gripper

g = stretch_body.stretch_gripper.StretchGripper()
g.startup()

g.trajectory.add(0, 0)
g.trajectory.add(3, 50)
g.trajectory.add(6, 0)
g.follow_trajectory()
time.sleep(6)

g.trajectory.clear()
g.stop()