void LMSM::simulate_initial_state(VectorView state)const {
    check_dim(state);
    state[0] = rnorm(initial_level_mean_, sqrt(initial_state_variance_(0,0)));
    state[1] = rnorm(initial_slope_mean_, sqrt(initial_state_variance_(1,1)));
    state[2] = slope_->mu();
}
 void change_dim(fixed::dimensions<Rows, Cols> d) { check_dim(d.num_rows(), d.num_cols()); }