Refactored random map generator system and added several terrain generators.

Created a global list to track base turfs for explosions/shuttle moves.
Remaps the asteroid to be a moonlet. Tidies up some references to 'asteroid', removes moonbase from the accessible z level list.

code/game/gamemodes/events/black_hole.dm

@@ -24,8 +24,11 @@
 			qdel(M)
 		for(var/obj/O in orange(1,src))
 			qdel(O)
+		var/base_turf = get_base_turf(src.z)
 		for(var/turf/simulated/ST in orange(1,src))
-			ST.ChangeTurf(/turf/space)
+			if(ST.type == base_turf)
+				continue
+			ST.ChangeTurf(base_turf)
 
 		sleep(6)
 		grav(10, 4, 10, 0 )
@@ -84,5 +87,6 @@
 	//Destroying the turf
 	if( T && istype(T,/turf/simulated) && prob(turf_removal_chance) )
 		var/turf/simulated/ST = T
-		ST.ChangeTurf(/turf/space)
-	return
+		var/base_turf = get_base_turf(src.z)
+		if(ST.type != base_turf)
+			ST.ChangeTurf(base_turf)