SPECTRAL_CLEAN FFT

          [BUNDLES\]SPECTRAL_CLEAN FFT

    Clean the Fourier Transform along the spectral axis of the DIRTY  cube,  and
    transform back the Clean components to retrieve the per-channel Clean compo-
    nents. The final Clean image is produced using UV_RESTORE.

    The  usual Clean convergence parameters apply, although CLEAN_NKEEP may need
    small adjustment because of the non-positive nature of the signal.

    The Fourier Transform leads to a Complex valued data set, whose  Hermiticity
    is  not  considered. Real and Imaginary parts are cleaned separately, so the
    method is about twice slower than a standard Clean.