Implement nearest_neighbour_mut

[joverwey]

The nearest_neighbour() function returns an immutable reference to and existing node.
Nodes with data associated with it, like GeomWithData<[f32;3], usize>, cannot be modified inplace.
Instead one has to use pop_nearest_neighbor, change the data, and then reinsert it.

An use example case for this is an rtree that keeps visitation counts for a set of positions.

[rmanoka]

This is by design as we don't want the geometry to change. Try GeomWithData<G, Cell<T>> . Then you can replace the inner T as per Cell semantics (incl. replace with immutable ref. if T is copy; usize is)

[grovesNL]

Does locate_in_envelope_intersecting_mut have the same issue? It seems like it allows us to change the geometry, but the tree wouldn't know that the envelope haw changed.