AP_AHRS : change initialisation requirements for EKF and use dynamic method
AP_NavEKF : Improved stability of Z accel bias
AP_NavEKF : Bug fix for height drift due to timer wrap-around
AP_NavEKF : Changed default IMU bias process noise to use smallest value
AP_NavEKF : fixed bug in height limit range
AP_NavEKF : fixed bug in velocity rate of change filter

This bug meant the velocity rate of change used to scale the GPS measurement variances was noisy and too small