TRANSFORM FFT

        [CALIBRATE\]TRANSFORM FFT  FileIn FileOut [Nchan]

    Compute the Fourier Transform of FileIn along the  Frequency/Velocity  axis.
    The direction of the Fourier Transform depends on the nature of FileIn.

    If  FileIn is a standard Data Cube, a direct Fourier transform is performed,
    that leads to 4-D cube as FileOut, in  order  (position,position,complex,ve-
    locity).  Its  3rd  axis  has  2  pixels that contain the Real and Imaginary
    parts.  The number of channels is extended to the  nearest  integer  of  the
    form 2^n 3^p 5^q where p and q are less or equal to 2.

    Conversely,  if  FileIn  is such a 4-D cube, an inverse Fourier transform is
    performed, leading to a (position,position,velocity) cube  as  FileOut.  Op-
    tionally,  Nchan can specify how many channels are actually retained in this
    case.