EKF: Prevent timestamp jitter failing GPS data arrival check (#462)
EKF: Prevent timestamp jitter failing GPS data arrival check (#462)

This fixes a error condition that occurs if _time_last_gps is greater than _time_last_imu.
By checking time stamps at the fusion time horizon, we guarantee that this cannot happen because all observations must have a time stamp smaller or equal to _imu_sample_delayed.time_us before they are retrieved from the buffers.
PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl,PX4/ecl