AP_NavEKF : More rapid wind estimation on launch without airspeed sensor

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.