The unit called FullSpectrum takes an input data type containing a spectrum
and outputs a full-spectrum type, restoring it to its full width if it
was narrow-band and to two-sided form if it was one-sided. Narrow-band
spectra are made wide-band by inserting zeros. Two-sided-ness is restored
assuming the spectrum came from a real data set, so that the negative frequencies
hold complex conjugate values of those at the positive frequencies.