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/modules/mob/dead/observer/observer.dm

@@ -107,7 +107,7 @@ GLOBAL_VAR_INIT(observer_default_invisibility, INVISIBILITY_OBSERVER)
 		if(turfs.len)
 			T = pick(turfs)
 		else
-			T = locate(round(world.maxx/2), round(world.maxy/2), ZLEVEL_STATION_PRIMARY)	//middle of the station
+			T = SSmapping.get_station_center()
 
 	forceMove(T)