AP_Airspeed: fixed saving of airspeed ratios
Rover: use L1 controller for navigation

this uses the AP_L1_Control library for rover navigation
Rover: don't use throttle_nudge in steering mode
Rover: send y accel in m/s/s as nav_pitch

this allows for easier tuning in MissionPlanner
SITL: reduce the minimum accel and gyro noise

on APM2 and PX4 we do a lot of smoothing of accels and gyros, so a 2
bit error isn't a good representation of the min noise
Rover: added STEERING_LEARN parameter

when enabled (set to 1) it will learn TURN_CIRCLE based on the
demanded and actual lateral acceleration