Provide is_tracking status for Dynamixel joints

[hello-binit]

The stepper joints will report whether the joint is tracking a given command via the "is_mg_moving" flag in the stepper status dictionary. This is useful when writing code for the robot, because it tells you when a motion has completed and/or if the joint is stalled against an obstacle in the environment (unable to push through to reach the commanded goal). The dynamixel joints don't report similar information, even though it is available (https://emanual.robotis.com/docs/en/dxl/x/xl430-w250/#moving-status) and is used internally in wait_until_at_setpoint():

stretch_body/body/stretch_body/dynamixel_hello_XL430.py

Line 395 in e7bb48f

if self.motor.get_moving_status() & (1 << 1) == 0:

Aside: I find the dynamixel's docs a bit confusing since bit 0, 1, and 3 all seem to report the same information. There may be some minute differences, so it's worth running some experiments to clarify and document what each bit does.