Fixes gas entropy calculation + engine pump/injector flow imbalance 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. Fixes #6166

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.

Also collected some data from testing the SM:
SM power - Stable Temperature - O2/TX %
60 kW - 179 K - 0/0
120 kW - 270 K - 0.3/0.3
180 kW - 390? K - 1/0.5

I wanted to collect some more data but just ran out of time. Plan on using this to adjust POWER_FACTOR later.

Also updated rad collector meters to be read by examining.