ConnectedDimensions

Stacks dimensions on top of one another. If you fly up you'll go from Nether->Overworld->End. Going down goes the other direction. Players will trigger chunk loading. All other entities will teleport if the destination is already loaded.

Changed world generation to remove the bedrock from the bottom of the overworld. The nether generation has more changes, not only to remove the bedrock ceiling but also to fill the entire build range. It thins out the higher Y levels because just falling on to the nether roof was just a bit too boring.