Compute the smoothed estimation of the parameters theta and P.
kalman_smoothing(X, y, theta1, P1, Q = 0, sig = 1)a list containing theta_arr and P_arr, the smoothed estimation of
the parameters theta and P.
the explanatory variables
the time series
initial theta
initial P
(optional, default 0) covariance matrix of the state noise
(optional, default 1) variance of the spate noise