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/modules/random_map/noise/desert.dm

@@ -0,0 +1,40 @@
+/datum/random_map/noise/desert
+	descriptor = "desert"
+	smoothing_iterations = 3
+
+/datum/random_map/noise/desert/replace_space
+	descriptor = "desert (replacement)"
+	target_turf_type = /turf/space
+
+/datum/random_map/noise/desert/get_map_char(var/value)
+	return "<font color='#[value][value][value][value][value][value]'>[pick(list(",",".","'","`"))]</font>"
+
+/datum/random_map/noise/desert/get_appropriate_path(var/value)
+	var/val = min(9,max(0,round((value/cell_range)*10)))
+	if(isnull(val)) val = 0
+	switch(val)
+		if(0 to 1)
+			return /turf/simulated/floor/beach/water
+		else
+			return /turf/simulated/floor/beach/sand/desert
+
+/datum/random_map/noise/desert/get_additional_spawns(var/value, var/turf/T)
+	var/val = min(9,max(0,round((value/cell_range)*10)))
+	if(isnull(val)) val = 0
+	switch(val)
+		if(2 to 3)
+			if(prob(60))
+				var/grass_path = pick(typesof(/obj/structure/flora/grass)-/obj/structure/flora/grass)
+				new grass_path(T)
+			if(prob(5))
+				var/mob_type = pick(list(/mob/living/simple_animal/lizard, /mob/living/simple_animal/mouse))
+				new mob_type(T)
+		if(5 to 6)
+			if(prob(20))
+				var/grass_path = pick(typesof(/obj/structure/flora/grass)-/obj/structure/flora/grass)
+				new grass_path(T)
+		if(7 to 9)
+			if(prob(60))
+				new /obj/structure/flora/bush(T)
+			else if(prob(20))
+				new /obj/structure/flora/tree/dead(T)