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/mapping/space_management/zlevel_manager.dm

@@ -24,7 +24,7 @@
 	// TODO: sleep here if the Z level needs to be cleared
 	var/datum/space_level/S = new z_type(new_z, name, linkage, traits)
 	z_list += S
-	return new_z
+	return S
 
 /datum/controller/subsystem/mapping/proc/get_level(z)
 	. = z_list[z]