AP_AutoTune: lower min tau to 0.2
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.
AP_NavEKF: start with a wind estimate of 3m/s, when no direct measurement

this will cope better with users with low roll/pitch gains, to ensure
they get enough control on takeoff

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
AP_AHRS: enable EKF wind estimate with no airspeed sensor

EKF can now estimate with IMU and GPS only

Pair-Programmed-With: Paul Riseborough <p_riseborough@live.com.au>
AP_Vehicle: added autotune_level to fixed wing parms
APM_Control: added support for AUTOTUNE_LEVEL

10 levels of tune, for what type of tune the user wants
Plane: added AUTOTUNE_LEVEL parameter
AC_WPNav: set_wp_destination to use current target

Previously we used the projected stopping point if the vehicle had not
reached the destination but this could lead to large jumps in target
position
Copter: clear RC in overrides on GCS failsafe.
AC_PosControl: get_accel_target returns const ref
Drift: get velocity using const reference