PID-loop tutorial

I use the LOG command output data to "throttle_log", it goes into a file called "throttle_log.txt" which I then rename to something like "throttle.001.log" in the OS. not sure if I can rename it within kOS. Anyways, the code examples have all been tested now.
In kOS programs I've writen in the past, I've added the current altitude to the g calculaton, like so:
```lock g to ship:body:mu / ((ship:body:radius+ship:altitude)^2).```
It doesn't make a big difference on big planets but it really matters on moons where the altitude can be significant compared to the radius.