[BUNDLES\]SPECTRAL_CLEAN MULTI Ns1 [Ns2 ...]
Clean a (spectrally) smoothed version of the data cube, then start from
this result to continue Cleaning with the original spectral resolution.
To be implemented: the method can be generalized to more than 1 smooth-
ing scale, by performing the smoothing in descending order (Ns1 > Ns2 >
... > 1). This may allow to catch signals that have widely different
widths.