Changes in Layouts , Addition of new files

[Sakshi-2797]

This contribution includes an alternate cpp implementation of the "optimize_layout_euclidean" function in layouts.py. The changes includes a faster cpp implementation of the optimization function by introducing separate files - umap_extend.cpp, xsrandom.cpp,xsrandom.hpp.

[lmcinnes]

This looks like impressive work, but for my own sake I would like to keep this version of umap as a python only library (for ease of maintenance and packaging). I would encourage you to build this work into a fork that can be a package you maintain (fast_umap perhaps?). You could roll in your spectral improvements there as well, and other speedups as you see fit.