Minor fixes and download stellar spectrum

[nichollsh]

• Now includes data for sulfur species - closes Add data for S2, SO2, and other gases #32
• Updates to plots, so that the colours are consistent with PROTEUS and AGNI

Now downloads stellar spectra from here: https://osf.io/8r2sw/, rather than using precomputed files in the spectral_files folder. This is important because the old files were outdated and not in a format consistent with what Mors produces. They were in the SOCRATES format with SI units and a specific header.

The spectra are now converted into the SOCRATES format by JANUS when they are inserted into the spectral file. This is done with PrepareStellarSpectrum in the same manner as in PROTEUS. The files produced by mors.SpectrumWrite can then be parsed appropriately.

As a result, the solar spectrum used by JANUS by default is slightly different to the previous one. The test reference values have been updated accordingly, but they only differ by a few decimal places.

[lsoucasse]

Great! Things are starting to fall into place. It is much better to discriminate spectral files associated with the atmosphere and spectral files associated with the star.

I am wondering if all these stellar spectra should better fit into Mors but I think for now it is OK.

[nichollsh]

Thanks!

Yes, it would make more sense if they were downloaded by MORS instead of by JANUS. The spectra are downloaded to the shared FWL data folder regardless, but maybe the functions themselves should be moved.