It will first check to see if the turf is set, and if not it will still use the z-level default.
This means we can mark areas as having a base of space to override the z-level base of asteroid that we're using.
As a result, we can ensure the bombs, shuttles moving, etc. leave behind an space turf by defining shuttles, solars/"external" areas as using space as a base.
Areas will need to be updated to check with base_turf = /turf/space and references to the old get_base_turf proc updated so all calls (bombs, Nar'Sie, deconstruction, etc.) have the correct effect, but it's tired and I want to commit this now so I'm committed to finishing it tomorrow.
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.