Allow construction of distance-based weights directly from KDTree/BallTree

[ljwolf]

Users sometimes will spend a lot of computation building KDTree/BallTrees that they they want to use in spatial/graph statistics. It would be nice if we can allow Graph.build_distance_band(), Graph.build_knn() (and thus likely Graph.build_kernel()) to accept the pre-built trees.

Fortunately, tree.data usually contains the input points, so we can generally rely on having both the tree and its input data. This means it's easy to adapt the patterns from W for these weight types.