Apply RTS smoother to EKF #317

Thanks for your awesome work, I have some questions about applying RTS smoother to EKF

1. I use the acc and gyr signal to estimate the attitude/pose with EKF, if it possible to apply rst smoother to this？There is no rts_smoother in EKF.py
2. I noticed the implementation of rts_smoother in kalman_filter.py. There are four input params, Xs(station/mean), Ps(uncertainties/variance), Fs(state transition), Qs(noise). As we known, Kalman-type filters have two steps, Predict and Update, these two steps both update the Xs and Ps, so which step's output should be used for rts_smoother？

Looking forward to your reply！