Tweaks plasma thrusters (#2797)

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## About The Pull Request
Changes how plasma thrusters calculate the maximum amount of fuel they
can store. Instead of using the heater's volume, they now use an amount
of moles based on standard conditions for plasma thrusters - if you cool
down your plasma or refit your ship to more efficiently fill the
thrusters than with a standard gas pump, you can still get numbers over
100% (just like you can with ion engines if you directly connect them to
a grid).
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## Why It's Good For The Game
Plasma thrusters showing like 180% with a default setup hurts my soul
and is also pretty unintuitive. With these changes, you can still tweak
your thrusters to be more efficient - for default plasma thrusters
though, this makes their display more useful.
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## Changelog

:cl:
tweak: Plasma thrusters now have a more sensible fuel readout
/🆑

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code/game/machinery/shuttle/shuttle_heater.dm

@@ -102,7 +102,10 @@
 	var/datum/gas_mixture/air_contents = use_tank ? fuel_tank?.air_contents : airs[1]
 	if(!air_contents)
 		return
-	return air_contents.return_volume()
+	//Using the ideal gas law here - the pressure is 4500 because that's the limit of gas pumps, which most ships use on plasma thrusters
+	//If you refit your fuel system to use a volume pump or cool your plasma, you can have numbers over 100% on the helm as a treat
+	var/mole_capacity = (4500 * air_contents.return_volume()) / (R_IDEAL_GAS_EQUATION * T20C)
+	return mole_capacity
 
 /obj/machinery/atmospherics/components/unary/shuttle/heater/proc/update_gas_stats()
 	var/datum/gas_mixture/air_contents = use_tank ? fuel_tank?.air_contents : airs[1]