PID Autotune Library
Contains the Autotune algorithm. It should be called once every loop() during the autotune prodecure. It will start the Autotune the first time it is called. Most of the time it will just return without doing anything. It will decide when the output is stepped as well as when the tuning process is done.
0 if autotune still needs to run, 1 if autotune has completed