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.