Compute the autocovariance function from the ARMA parameters
        over the integers range(num_autocov) using the spectral density
        and the inverse Fourier transform.

        Parameters
        ----------
        num_autocov : scalar(int), optional(default=16)
            The number of autocovariances to calculate

def autocovariance(self, num_autocov=16):
        """
        Compute the autocovariance function from the ARMA parameters
        over the integers range(num_autocov) using the spectral density
        and the inverse Fourier transform.

        Parameters
        ----------
        num_autocov : scalar(int), optional(default=16)
            The number of autocovariances to calculate

        """
        spect = self.spectral_density()[1]
        acov = np.fft.ifft(spect).real

        # num_autocov should be <= len(acov) / 2
        return acov[:num_autocov]