cepstrum — cepstrum calculation
fresp = cepstrum(w,mag)
positive real vector of frequencies (rad/sec)
real vector of magnitudes (same size as w)
complex vector
fresp = cepstrum(w,mag) returns a frequency response fresp(i)
whose magnitude at frequency w(i) equals mag(i) and such
that the phase of freq corresponds to a stable and minimum phase
system. w needs not to be sorted, but minimal entry should not be
close to zero and all the entries of w should be different.