Tuning changes

http://brettbeauregard.com/blog/2011/04/improving-the-beginner%E2%80%99s-pid-tuning-changes/

This allows us to change P, I, and D values on the fly without undesirable side effects.
In go it's idiomatic to name getters without `Get` as a prefix. So we should just use `PID` for the name of this method.