Gas entropy

This PR expands on the fix from 9fea39dea216ed6fa1f1aa167d5c16c582a5e9ee.

* Atmos control computers can set the air injector flow rate.
* The SM engine gets it's own atmos control computer type that sets external pressure bounds instead of internal, as appropriate for the engine core which is not an air storage tank. Players now have direct control over the engine core pressure.
* New high-power engine vent pump type whose max flow rate matches the air injector max flow rate.