init_kf computes the starting values x0 and P0 by generalized least
squares using the first n observations.
init_kf(mdl, z = NULL, n = 0)A list with components:
initial state vector estimate
covariance matrix of the initial state estimate
an object of class ssm.
optional time series if it differs from model series.
integer, number of observations used to estimate the initial conditions. If n < d (dimension of state vector), it defaults to length(z).