Just a tiny change adds a <reference> input to FGDigitalFilters. This is nice to...

Just a tiny change adds a <reference> input to FGDigitalFilters. This is nice to have to calculate differences of signals using gain-filters.
Old calculation was
output = input * gain
and is now
output = (input-reference) * gain
Note: the PI(D) controller use (reference-input) which effectively reverses the sign. Our notation was picked for backwards compatibility to not break the myriads of <filter> elements currently existing.

All the rest is some code cleanup.