The teleport block must be on a side, not a corner and all blocks inside the frame must not be solid, opaque or impaasible.

…mes.

Outstanding issues:
- Teleporters work now, but at least in my save, existing ones need to be completely rebuilt.
- The frame blocks consistently trigger the multiblock validation, but the teleporter blocks themselves don't always trigger it...
- Shared frames do not work.
- Multiple teleporter blocks can be added to one frame.
- I needed to change Structure.java to allow for flat structures and I'm not sure what affect that will have on other uses of the class. It seems like it will just mean that precheck in CuboidStructureValidator will get called more often.

…rter frames."

This reverts commit c68c432