Transform iddata objects from frequency to time domain
dat = ifft(Datf)
ifft transforms a frequency-domain iddata object
to the time domain. It requires the frequencies on Datf to
be equally spaced from frequency 0 to the Nyquist frequency. This
means that if there are N frequencies in Datf and
the sample time is Ts, then
Datf.Frequency = [0:df:F], where F is pi/Ts if N is
odd and F = pi/Ts*(1-1/N) if N is
even.