Refactor weather to use Z traits, assorted related cleanup (#34633)

* Add a proc for getting the station center

* Add a couple of comments to ZTRAIT defines

* Remove unused global_map list

* Refactor weather to use the trait system

* Un-hardcode the transit z-level

* Use Z traits to determine Portal Storm event areas

* Fix loading away missions containing anything that reads traits

code/game/area/areas.dm

@@ -384,9 +384,9 @@ GLOBAL_LIST_EMPTY(teleportlocs)
 			icon_state = "blue-red"
 	else
 		var/weather_icon
-		for(var/V in SSweather.existing_weather)
+		for(var/V in SSweather.processing)
 			var/datum/weather/W = V
-			if(src in W.impacted_areas)
+			if(W.stage != END_STAGE && (src in W.impacted_areas))
 				W.update_areas()
 				weather_icon = TRUE
 		if(!weather_icon)