* Remove redundant null checks for atmos
* Remove unnecessary nullability parameter from AtmosphereSystem.GetGridAtmosphere
* Remove more nullability markers
* Bring back null checks for gas tanks
* Remove null checks from GasMixture.Merge and TileAtmosphere.AssumeAir
Co-authored-by: Vera Aguilera Puerto <6766154+Zumorica@users.noreply.github.com>