Re-adds and reworks fusion. (#37384)

add: Fusion is back
add: Fusion now happens with large quantities of hot plasma and CO2, consuming all the plasma rapidly and producing large amounts of energy. Other gases can also effect the performance of the reaction.
add: Fusion now produces radiation. Be careful around reactors.
fix: fixes fusion dividing by zero and destroying everything
code: Gas reactions now have access to where they are occurring
fix: Prevents some other reactions from producing matter from nothing

code/modules/reagents/chemistry/reagents/food_reagents.dm

@@ -438,7 +438,7 @@
 	if(hotspot)
 		var/datum/gas_mixture/lowertemp = T.remove_air(T.air.total_moles())
 		lowertemp.temperature = max( min(lowertemp.temperature-2000,lowertemp.temperature / 2) ,0)
-		lowertemp.react()
+		lowertemp.react(src)
 		T.assume_air(lowertemp)
 		qdel(hotspot)
 

code/modules/reagents/chemistry/reagents/other_reagents.dm

@@ -146,7 +146,7 @@
 		if(T.air)
 			var/datum/gas_mixture/G = T.air
 			G.temperature = max(min(G.temperature-(CT*1000),G.temperature/CT),0)
-			G.react()
+			G.react(src)
 			qdel(hotspot)
 	var/obj/effect/acid/A = (locate(/obj/effect/acid) in T)
 	if(A)

code/modules/reagents/chemistry/reagents/pyrotechnic_reagents.dm

@@ -273,7 +273,7 @@
 			var/datum/gas_mixture/G = T.air
 			if(G.temperature > T20C)
 				G.temperature = max(G.temperature/2,T20C)
-			G.react()
+			G.react(src)
 			qdel(hotspot)
 
 /datum/reagent/firefighting_foam/reaction_obj(obj/O, reac_volume)