Lift and arm trajectories

[hello-binit]

This PR makes three methods available for the arm and lift joints:

• 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

The previous work on trajectory management classes and waypoint trajectory RPCs is used here to provide a high level interface to commanding waypoint trajectories. This PR includes unit tests of the new methods, and a small example program is reproduced here:

import stretch_body.arm

a = stretch_body.arm.Arm()
a.motor.disable_sync_mode() # not necessary when using arm from the Robot class
a.startup()

a.trajectory.add(t_s=0, x_m=0.1)
a.trajectory.add(t_s=3, x_m=0.2)
a.trajectory.add(t_s=6, x_m=0.15)
a.follow_trajectory()
print('Executing {0}'.format(a.trajectory))
time.sleep(6)

a.trajectory.clear()
a.stop()