Cause was faulty logic in datum/gas_mixture/mimic() that was leaking a null value into some multiplication math. It's still wrong, but at least it keeps the air too cold instead of making it literally infinitely hot. Fixes #7691 Fixes #4740 Fixes #8166
Fixes turfs losing air too slow next to space turfs when the adjacent turfs number is low.
group_multiplier
Moved gas_mixture.dm to the datums directory. Deleted FEA.