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.

Package: tsa