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/automata/caves.dm

@@ -0,0 +1,52 @@
+/datum/random_map/automata/cave_system
+	iterations = 5
+	descriptor = "moon caves"
+	wall_type =  /turf/simulated/mineral
+	floor_type = /turf/simulated/floor/plating/airless/asteroid
+	target_turf_type = /turf/unsimulated/mask
+	var/mineral_sparse =  /turf/simulated/mineral/random
+	var/mineral_rich = /turf/simulated/mineral/random/high_chance
+	var/list/ore_turfs = list()
+
+/datum/random_map/automata/cave_system/get_appropriate_path(var/value)
+	switch(value)
+		if(DOOR_CHAR)
+			return mineral_sparse
+		if(EMPTY_CHAR)
+			return mineral_rich
+		if(FLOOR_CHAR)
+			return floor_type
+		if(WALL_CHAR)
+			return wall_type
+
+/datum/random_map/automata/cave_system/get_map_char(var/value)
+	switch(value)
+		if(DOOR_CHAR)
+			return "x"
+		if(EMPTY_CHAR)
+			return "X"
+	return ..(value)
+
+/datum/random_map/automata/cave_system/revive_cell(var/target_cell, var/list/use_next_map, var/final_iter)
+	..()
+	if(final_iter)
+		ore_turfs |= target_cell
+
+/datum/random_map/automata/cave_system/kill_cell(var/target_cell, var/list/use_next_map, var/final_iter)
+	..()
+	if(final_iter)
+		ore_turfs -= target_cell
+
+// Create ore turfs.
+/datum/random_map/automata/cave_system/cleanup()
+	var/ore_count = round(map.len/20)
+	while((ore_count>0) && (ore_turfs.len>0))
+		if(!priority_process) sleep(-1)
+		var/check_cell = pick(ore_turfs)
+		ore_turfs -= check_cell
+		if(prob(75))
+			map[check_cell] = DOOR_CHAR  // Mineral block
+		else
+			map[check_cell] = EMPTY_CHAR // Rare mineral block.
+		ore_count--
+	return 1