Save compatibility across physics/scale cleanups

Even simply cleaning up the way various constants are used means that the game scale changes.

If we ever reshuffle the Solar system (which I definitely would prefer to do just to reduce the count of crimes against science) or just make a sane scale hierarchy (imagine orbiting a portal that orbits Moon — and you cannot realistically enter a Moon orbit right from the Solar orbit without overclocking — I think that would give a better space piloting feel), the saves will once again have a problem.

Maybe it is a good idea to add physics version to saves? So that if you load a save with compatible mass/distance scale, the positions are loaded, too — otherwise you are spawned at the portal used for entering the system (which we would have to keep, and default to shortest hopset from the initial system).