Moves assert_gas() into a define it should always have been, speeding up atmos slightly

code/__DEFINES/atmospherics.dm

@@ -175,4 +175,12 @@
 #define LAVALAND_EQUIPMENT_EFFECT_PRESSURE 50 //what pressure you have to be under to increase the effect of equipment meant for lavaland
 #define LAVALAND_DEFAULT_ATMOS "o2=14;n2=23;TEMP=300"
 
-#define THERMAL_ENERGY(gas) (gas.temperature * gas.heat_capacity())
+#define THERMAL_ENERGY(gas) (gas.temperature * gas.heat_capacity())
+
+#define ADD_GAS(gas_id, out_list)\
+	var/list/tmp_gaslist = GLOB.gaslist_cache[gas_id];\
+	out_list[gas_id] = tmp_gaslist.Copy();
+
+//ASSERT_GAS(gas_id, gas_mixture) - used to guarantee that the gas list for this id exists in gas_mixture.gases.
+//Must be used before adding to a gas. May be used before reading from a gas.
+#define ASSERT_GAS(gas_id, gas_mixture) if (!gas_mixture.gases[gas_id]) { ADD_GAS(gas_id, gas_mixture.gases) };