Fixes gas entropy calculation, another engine issue

Adjusted the gas entropy calculation so that the source gas mixture being at a higher temperature than the destination doesn't impede gas flow, but instead makes gas flow easier, like you might expect. Added a bunch of comments to the specific_entropy_gas() proc explaining it's significance for atmos machines in case anyone wants to tweak it.

While testing this, I came across another issue with the SM engine, which was that the maximum flow rate out of the core through the vent pump was 2500 L/s, while the maximum flow rate into the core through the air injector was 200 L/s. Added a quick fix for this by limiting the flow rate of vent pumps to 200 L/s. A more thorough fix would probably involve implementing a way for players to control/limit the flow rate on both devices.