Optimization of tree-traversal in mlpack

by Andrew for mlpack: scalable C++ machine learning library

The goals of this project are twofold: A) Speed-up dual-tree traversal algorithms in mlpack. 1) Define a set of benchmarks. 2) Improve the speed of the tree traversal as measured by these benchmarks. 3) Document, test, and support this code. B) Implement various trees in a manner consistent with the current API. 1) X-tree 2) R*-tree 3) R-tree 4) Hilbert-R-tree 5) UB-tree 6) M-tree