v1.9 LSST Baseline throughputs

[stevet40]

This solves #46 for now.

I've added the v1.9 total throughputs for LSST from this repo:
https://github.com/lsst/throughputs/tree/main/baseline

I converted the wavelength column to Ångstroms. I added some comments to the top of the files to describe my understanding of these transmission curves. I think it's possible the recommended curves will change at some point, but hopefully these are okay for now. Let me know if you think I should add more detail in the comments at the tops of the files.

[bd-j]

Hi @stevet40, thanks for this.

The comments look good. I think in general we'll only want to keep a 'most current' version so I could imagine removing the 1_9 from the filenames and keeping that info in the comments (as for most other filter sets). But perhaps if frequent turnover is expected then tracking the version explicitly is better.

These are very high resolution! Not sure how necessary it all is, maybe time to actually implement #8

[stevet40]

Yeah, maybe annoying to have the 1_9 in the filenames, so no objection to taking those out! The wavelength grid is what is provided, and is indeed quite dense. For ugr it definitely looks like overkill. The izy have a lot more atmospheric features, and the y particularly is very spiky!

[bd-j]

LGTM

[bd-j]

Thanks @stevet40