Get arm rotation PID working

[n-arms]

Write code to rotate the arm to a specific angle. The arm subsystem should operate in two modes, PID driven to an angle (should be used all the time), and raw input driven by motor speed (if PID/the encoder breaks for whatever reason).

Raw input mode

• arm keeps sets the motor speed to the given speed

  PID mode

• encoder readings need to be accurate and consistent access power cycles, moving the arm while the robot is off, etc
• arm keeps track of target angle
• arm uses PID + maybe gravity feed forward to rotate to the correct angle
• make sure the arm motor has a current limit so it doesn't burn out

[n-arms]

In the room today we tested most of this branch: we desketchified soft stops + encoder readings. We still have to test PID code to tune the appropriate constants.

[n-arms]

completely working!