Manage different trees 2

Alternative implementation to #14. Instead of handling multiple trees in one instance of `CCHMapClusterController`, this implementation allows multiple `CCHMapClusterController`s to use a single `MKMapView` instance.

Pros:
- `CCHMapClusterController`s are independent of each other -> get parallelism for free, can control settings independently
- Less additional code/complexity in `CCHMapClusterController`

Cons:
- Implementation in `CCHMapViewDelegateProxy` is perhaps a bit too cute - relies on Objective-C runtime. There might be edge cases where implementation fails