MAINT merge KDTree and BallTree into a single module

Saves ~40kLOC of generated C code.

Author: larsmans

.gitattributes

@@ -10,6 +10,7 @@
 /sklearn/linear_model/sgd_fast.c -diff
 /sklearn/metrics/pairwise_fast.c -diff
 /sklearn/neighbors/ball_tree.c -diff
+/sklearn/neighbors/binary_tree.c -diff
 /sklearn/neighbors/dist_metrics.c -diff
 /sklearn/neighbors/kd_tree.c -diff
 /sklearn/svm/liblinear.c -diff

sklearn/neighbors/__init__.py

@@ -3,8 +3,7 @@
 algorithm.
 """
 
-from .ball_tree import BallTree
-from .kd_tree import KDTree
+from .binary_tree import BallTree, KDTree
 from .dist_metrics import DistanceMetric
 from .graph import kneighbors_graph, radius_neighbors_graph
 from .unsupervised import NearestNeighbors