Copter: added AUX switch option for enable/disable EKF
AP_AHRS: added set_ekf_use()
Copter: use GPS AIRBORNE_4G
AP_AHRS: use the right DCM matrix when EKF not enabled
AP_NavEKF: copter specific dtIMUAvg
AP_NavEKF: use INS delta_time
AP_Math: make to_euler() const
AP_AHRS: fill in _dcm_matrix whenever EKF started

this ensures get_dcm_matrix() always returns valid data
AP_AHRS: start EKF 5 seconds after getting GPS lock