* Interpolate ki/kp for steering PID loop
Very much needed for the Volt port: car ping-pongs with low kp
on high speeeds, and the loop is unstable with high kp on
low speeds.
Also, removes "number or array?" logic from PIController,
now that all the callers use interpolation ofr ki/kp.
* Pass speed to steering PID loop for ki/kp interpolation
* Remove unused numbers import
old-commit-hash: 93f55f3ccf
