Fix a crash in ambient sound biome checking when the player is halfway between level 2 and level 3. Also add API to puzzle chests

This fixes issue https://github.com/FaceDeer/dfcaverns/issues/39

df_caverns/shared.lua

@@ -28,6 +28,7 @@ df_caverns.register_biome_check = function(func)
 	table.insert(get_biome_at_pos_list, func)
 end
 df_caverns.get_biome = function(pos)
+	pos = vector.round(pos)
 	local heat = minetest.get_heat(pos)
 	local humidity = minetest.get_humidity(pos)
 	for _, val in pairs(get_biome_at_pos_list) do