[DNM] XGM gas mixtures

new, generic, non hardcoded gas mixtures ported from bay.
This PR will not be merged before #6644.
putting this up for feedback, map still needs to be fixed and other issues ironed out.

things to look at:
- [ ] fix fires appearing out of nowhere, strange temperature increases?
- [ ] fix superconductivity
- [ ] fix fire code in general
- [ ] look into making init_gas easier to use
I'd be all for this but it means that gas definition has to be somewhere outside of gases.dm and kinda messes up the modularity.