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/events/wizard/advanced_darkness.dm

@@ -13,4 +13,4 @@
 /datum/round_event/wizard/darkness/start()
 	if(!started)
 		started = TRUE
-		SSweather.run_weather("advanced darkness", ZLEVEL_STATION_PRIMARY)
+		SSweather.run_weather(/datum/weather/advanced_darkness)

code/modules/events/wizard/lava.dm

@@ -12,4 +12,4 @@
 /datum/round_event/wizard/lava/start()
 	if(!started)
 		started = TRUE
-		SSweather.run_weather("the floor is lava", ZLEVEL_STATION_PRIMARY)
+		SSweather.run_weather(/datum/weather/floor_is_lava)