serializability

This implementation is not serializable.

I would like to broadcast the rtree2 in spark. this requires me to have it serializable.
How could I add this functionality back in?

```
// deserialize the entries from disk (for example)
List entries = ...
// bulk load
RTree tree = RTree.maxChildren(28).star().create(entries);
```
how could something like this work in the case of a spark broadcast variable?