Single-Dish radio spectra and derived rotation measures that we got from
Shane O'Sullivan <shane.osullivan@dcu.ie> via Yan.

Actually, I pulled the data off of https://tinyurl.com/RMcatalog and
https://tinyurl.com/PolSpectrav1, which both pointed to google drive in
2022.

I've taken most of the column metadata from
https://github.com/CIRADA-Tools/RMTable where that exists.  See
bin/colstd-to-rd.py for how I did that.

https://github.com/CIRADA-Tools/PolSpectra

The latter gives an idea of the sort of data we have here.

The test data is the first five rows of the input fits:

stilts tpipe in=data/LoTSS_PolSpectra.fits cmd="head 5" out=sample.fits