* Support minimal dependencies in examples
* bins="auto" only supported for numpy >= 1.11
* viridis is default cmap in matplotlib 2
* Use signature instead of viridis cmap
Use a sparse matrix representation of the neighbors.
Re-factored the QuadTree implementation to avoid insertion errors.
Various fixes in the gradient descent schedule to get the Barnes Hut and exact solvers to behave more robustly and consistently.
Xiao Yuan