Add/correct/enhance spectral models

[landmanbester]

I don't think we currently have an implementation for the non-log scale polynomial spectrum in wsclean (i.e. the second one specified here. It looks like the log polynomial is there and I think/hope that is what crystalball uses but the "std" implementation does not match. It's an easy enough fix that I will try to get to but I thought I would just check whether we should replace base="std" with the correct parametrisation, or leave it in but print a big fat warning if someone calls it saying that it will be deprecated and/or is probably incorrect, or just leave it as is?