'Get the impulse response corresponding to our model.
Returns
psi : array_like(float)
psi[j] is the response at lag j of the impulse response.
We take psi[0] as unity.'

def impulse_response(self, impulse_length=30):

    sys = (self.ma_poly, self.ar_poly, 1)
    (times, psi) = dimpulse(sys, n=impulse_length)
    psi = psi[0].flatten()
    return psi
