this fix ensures the output throttle is never in the opposite direction from the desired-speed
there is a possibility that this could lead to rougher throttle response when the vehicle is transitioning from forward to backwards motion because the throttle response will immediately go to zero when the desired speed cross over zero
Throttle controller has these advantages over existing controller:
based on velocity in vehicle's forward-back axis rather than ground-speed
straight-forward PID controller using speed error as input
speed control acceleration limts
stop control slows vehicle smoothly
configurable filtering
Steer controller has these advantages over existing controller:
output scaled properly for skid-steering vehicles
layered P and PID controller for angular error and rate control
configurable filtering
