Upgrade PID Library to 1.2.1

[krkeegan]

This release adds a new feature call Proportional on Measure PonM which you can read about:

http://brettbeauregard.com/blog/2017/06/introducing-proportional-on-measurement/
http://brettbeauregard.com/blog/2017/06/proportional-on-measurement-the-code/

So, from my basic understanding. The benefits of PonM are most applicable to systems where the output affects the rate of change of the input. Heating elements seem to be the canonical example.

In our system, the velocity controller seems to be largely proportional to me. The output generally seems to directly affect the input. And in playing with the controller, I can largely get good results with little overshoot. Plus mild overshoot in velocity may actually be a slight benefit.

The position controller however is another thing, its output velocity affects the rate of change of the input, position. I suspect that this controller would greatly benefit from this feature.

[krkeegan]

I have accomplished this safely in 721c172.

The PonM is very much not helpful with the velocity PID. I still have hope that it will work well with the positional PID. All of these PID changes should be tested together, I should hopefully have a pull request by early next week.