Implement multi(-joined)-polarization channel interpolation

Radler supports already interpolation of frequency channels to deconvolve with fewer channels than during gridding. However, it did not support multiple polarizations when interpolating. For now, I've implemented this by fitting every polarization separately and interpolating the fitted curve. It's debatable if this makes sense to do for e.g. QU (because e.g. of fluctuations cause by non-zero rotation measure) but it could be used to clean I to other pol leakage and maybe there are more use-cases. Someone tried this feature (join pol + deconvolution channels_) and before this MR it just crashes.

Original issue: https://gitlab.com/aroffringa/wsclean/-/issues/180