Gravity APIs

[skyem123]

It would be useful to people writing other mods if this mod had an API for the stuff it does with gravity.

One API would be an optional interface with a method which can be applied to a world object (is that the same as a dimension?), so that the world's gravity can be reported to other mods, such as this one (and maybe another mod could actually use it to make changes to the world's gravity).

Another API that would be useful is one that would allow other mods to apply gravity effects to entities using the logic inside this mod, and maybe even a way for another mod to replace the strange anti-gravity logic in this mod with something more sane.