[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'm pretty sure you can remove all occurrences of this.  LINDA doesn't use it, so it's just an extra unnecessary multiplication all over the place.
Confirm, @Aranclanos?