Refactor Positions classes in wk backend

[fm3]

The wk backend has BucketPosition, VoxelPosition, CubePosition. Does it really need all three? What members do these need? I’d say BucketPosition does not need to store voxel positions at all, but should rather just store the bucket x, bucket y, bucket z, and provide accessors for the rest. It should also provide from methods in a companion objects.
Also, the x-y-z tuples should be stored as Vec3Int. Let’s see what we can transfer to VoxelPosition and CubePosition here.

Also, could Cuboid and BoundingBox could be merged? Cuboid is a mag-aware bounding box

Should they have additionalCoordinates?