Further generalization of trees to include cover trees

**Reported by rcurtin on 26 Sep 41833825 05:35 UTC**
There is an implementation of cover trees in Bill's `contrib/` code that should be taken and added to `core/tree/`.  We need to figure out how to make our own tree usage more generalized so that a user can either pass a `BinarySpaceTree` or a `CoverTree` to a method.
