Mostly ports /tg/'s method of seeding submaps into specific z-levels.
Due to the low number of submaps present, all of the PoIs are guaranteed to spawn. As more are added, this should fix itself.
* Settings for specific z-levels are defined in datum types, allowing the lists of Z levels to be automatically configured. This solves the problem of being unable to use the z-level #defines in associative lists.
* Converted the example and northern star maps to use the new system.