Add Low pass filter to increase Ziegler Nichols

[timdrysdale]

ZN - relies on parasitic filtering effect. Any minor non-linearities of the system that modify the system from standard second order into higher order (standard 2nd order system, theoretically, will never become unstable). Since we know system already non-linear somewhat, we rely on them to de-stabilise. Want to make that effect more pronounced - by making it third order by adding another pole - add a low-pass filter. Implement straightforward low-pass filter. Academic (Aristides) would like to experiment with this - so this suggests we need a new command which can be set e.g. via debug interface for some testing on a couple of disks over summer.

[timdrysdale]

implementation - can't filter in the ISR, but can update the count, and then run filter at start of main loop timestep code.