Add yaw rate PI controller.

This commit adds Matlab code for design of PI controller gains, output of yaw
rate measurement matrix, and firmware updates to implement PI yaw rate control.
No anti-windup is performed so we need to verify that steer torque does not
saturate. Crossover frequency was selected to be 0.1 Hz. Strangely, the Matlab
pidtune() function yields an integral only controller even though a PI
controller is requested.