Error with the continuum channels identification when the spectral windows are treated one after the other

[smaret]

For some sources and settings, for example SVS13 setting 3, the UV_PREVIEW command (which identifies line-free channels to extract the continuum) sometimes gives different results if the spectral windows are treated one after the other, or if they are treated one by one. In the case of SVS13 setting 3, no line-free channels are found when the spectral window 27 is treated after 25 (and the script returns an error), but it works fine when we treat only spectral window 27.

A simple workaround is to treat the spectral windows one by one, by setting the redu%spws array to a single value.

This is a minor issue. I'm logging it here for reference.