Frequency input for RMsynth_3D

[PaddyLeahy]

The default way that RMsynth_3D should read frequency information is from the FITS header. An externally supplied frequency list should be provided as a non-default option, since it is always preferable to get key metadata from the data object. In particular RMsynth_3D should provide support for the -TAB convention, in which an irregularly gridded axis has an associated FITS extension table giving the frequencies (for instance) corresponding to each channel. See Greisen et al (2006) DOI: 10.1051/0004-6361:20053818.