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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,velocity). 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 2n̂ 3p̂ 5q̂ 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. Optionally, Nchan can specify how many channels are actually retained in this case.