[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:
- [x] fix fires appearing out of nowhere, strange temperature increases?
- [ ] fix superconductivity
- [ ] fix fire code in general
- [ ] fix / redo overlays
- [ ] look into making init_gas easier to use
- [ ] fix map errors, convert everything to init_gas (oh god)
and, after talking in IRC, won't work for init_gas.