APM_Control: first version of APM_Control autotuning

this adds autotune to the roll/pitch controllers using a very simple
mechanism. The plan is that this provides a framework which Paul and
Jon will build upon.
Plane: added AUTOTUNE flight mode

this is just like FBWA, but with automatic roll/pitch tuning
DataFlash: reserve a range of msg types for autotuning logs
APM_Control: added dataflash logging to autotune
Plane: changed for APM_Control API
APM_Control: fixed a bug in autotune servo saturation check

its in degrees not centi-degrees
APM_Control: fixed saving of new rate gains
AP_NavEKF : Enable calc of wind velocity when not using airspeed sensing

This patch also cleans up the logic associated with use of the synthetic
sideslip measurement so that it can never be used for a non fly-forward
vehicle type

This patch adds functionality that initialises the wind-speed vector to the
reciprocal of the ground speed vector, and scaled to 6 m/s. On average this gives
a better initial wind velocity estimate on launch by assuming:

a) launch will be into wind
b) wind speed is equal to global average

It also helps prevent a headwind causing initial underestimation of airspeed
causing high autopilot gains and limit cycles on climb-out, until first
turn when the EKF is able to estimate the wind.
AP_NavEKF : Increased position gate default to reduce impact of accel errors

Flight testing with windup turns has shown that the position gate threshold
can be tripped with good GPS data causing position jerks. This increases the
initial GPS glitch rejection threshold to effectively 5m when using the
default POSNE_NOISE value of 0.5m.