generates the order n derivative of the gaussian window, centered at frequency f0
gn = gaussn(f0, n)
a positive scalar in ]0 0.5], the normalized center frequency in (Hz).
a positive integer; the order of derivation
a real row vector the coefficients of the nth order derivative.
The wavelet gn is real, but it is its analytic form that is synthetized first. The real part is then normalized by the analytical value of its energy (so, L2 normalization!)