Overpressure exploits

There are a number of creative exploits to get gassed or liquids to overpressure. They all seem to involve putting a separate element at low quantity to get in the way of. Since the game fundamentally lacks mixing or partial pressures, this seems implausible to fix. However, that’s not true. There is a hacky way:

Before outputting from a vent, check the pressure from the location where the element will be pushed to. That should approximate partial pressure nicely.

Possibly even better: check the average pressure of the same element within 5 reachable squares of the vent. Which might be technically easier and smooth out a lot of strange behaviors.

This second option may also be appropriate to give steam turbines some implementation sanity. Perhaps allow them to be rebalanced, since the exploits would be removed.

With the addition of reservoirs, I think now is a good time for this fix.