Arducopter: Tradheli changes to Alt_Hold controllers.
Arducopter: TradHeli

Make the Throttle Accel I-term into a leaky I-term to help prevent weird interactions with the Accel I-term.
Arducopter: Tradheli:
Creating variable rate filter for throttle output.
Merge branch 'ArduCopter-2.9' of https://github.com/R-Lefebvre/ardupilot into ArduCopter-2.9