Add new jupyter notebook tutorial for PIMU in stretch base

[hello-ag]

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[hello-binit]

@hello-ag Thanks for opening this PR! Here are answers to the questions you've left within the notebook:

• Question - any suggestions for checking PIMU?
  I'm not sure what this question is asking.

• Question: Maybe I could write about the function of the fan.
  This would be a good idea. You can include information about the on-board NUC and how temperature affects performance.

• Question: maybe we can find a better way to determine yrange here?
  I believe yrange is determined from calibration of the cliff sensors. You can investigate this.

• Question: Maybe more description of the various IMU sensor readings could be added.
  Agreed, this would be helpful. Some people may not know what roll/pitch/yaw refers to.

• Question: Any example of trigger_motor_sync I can use here?
  I recommend issuing motor commands to two steppers, and using push_command from the stepper class. Since trigger_motor_sync isn't automatically called, you'll find the motors won't move, and that the status reports that the motors are waiting for the motor sync to trigger. Then you can issue a trigger_motor_sync to see motion. Finally, you can explain how Robot.push_command handles all of this.

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I recommend you clean up some of the empty cells and check for grammar/spelling. Finally, I'd add a README to the doc/ folder which explains how to use the jupyter command to visualize a .ipynb notebook file.