Calculates Bispectrum [BISPEC] = bispec(Z,N); Input: Z Signal N % of coefficients Output: BiACF bi-autocorrelation function = 3rd order cumulant BISPEC Bi-spectrum Reference(s): C.L. Nikias and A.P. Petropulu 'Higher-Order Spectra Analysis' Prentice Hall, 1993. M.B. Priestley, 'Non-linear and Non-stationary Time series Analysis', Academic Press, London, 1988.