forecast(out, fc=1, Zt.fc=NULL, Tt.fc=NULL, Rt.fc=NULL,
Ht.fc=NULL, Qt.fc=NULL)
y_t = Z_t * alpha_t + eps_t (observation equation)
alpha_t+1 = T_t * alpha_t + R_t * eta_t(transition equation)
where eps_t ~ N(0,H_t) and eta_t ~ N(0,Q_t).
Dimensions of variables are: 'yt' p*n 'Zt' p*m or p*m*n 'Tt' m*m or m*m*n 'Rt' m*r or m*r*n 'Ht' p*p or p*p*n 'Qt' r*r or r*r*n