Iterative Explosions (#3838)

This PR replaces recursive explosions with a new spreading engine: iterative explosions.
Should behave similarly, but also be more reliable, potentially faster.

Iterative explosions also support simple-explosion-esque directional explosion sounds/shake, and will now traverse Z-levels if the explosion is strong enough.

Also changes playsound() to transmit sounds across Z-levels. Uses BYOND's 3rd coordinate on /sound to make sound sound above/below the player if they're using headphones. Removed for now, can't get Z-falloff working right.

Fixes #2199.

code/controllers/subsystems/initialization/atlas.dm

@@ -17,6 +17,8 @@ var/datum/controller/subsystem/atlas/SSatlas
 	var/map_override	// If set, SSatlas will forcibly load this map. If the map does not exist, mapload will fail and SSatlas will panic.
 	var/list/spawn_locations = list()
 
+	var/list/connected_z_cache = list()
+
 /datum/controller/subsystem/atlas/New()
 	NEW_SS_GLOBAL(SSatlas)
 
@@ -103,6 +105,8 @@ var/datum/controller/subsystem/atlas/SSatlas
 		var/obj/effect/landmark/map_data/marker = thing
 		marker.setup()
 
+	connected_z_cache.Cut()
+
 /datum/controller/subsystem/atlas/proc/get_selected_map()
 	if (config.override_map)
 		if (known_maps[config.override_map])