lp_distance: Fast \(l_p^p\) Distance Vector Computation
Description
Takes in a double matrix \(\mathbf{X}\), whose transpose \(\mathbf{X}^T\)
has \(N\) rows, and computes a vector recording all
\({N \choose 2}\) pairwise \(l_p^p\) distances of \(\mathbf{X}^T\),
ordered lexicographically.