'spectral density from MA polynomial representation for ARMA process
References
Cochrane, section 8.3.3'

def spdpoly(self, w, nma=50):

    mpoly = np.polynomial.Polynomial(self.arma2ma(nma))
    hw = mpoly(np.exp((1j * w)))
    spd = np.real_if_close((((hw * hw.conj()) * 0.5) / np.pi))
    return (spd, w)
