Access to high-dimensional fuzzy simplicial set

[pcamara]

I find for some applications (e.g. clustering) it is good to have access to the high-dimensional fuzzy simplicial set through the class UMAP. This can be easily implemented by storing graph as self.graph in the method UMAP.fit(). If you think this is a useful feature but are too busy with more urgent things, I will be happy to implement it through a pull request. Please let me know.

[lmcinnes]

I agree that this would be useful -- I'm busy doing some refactoring in a branch right now, but please feel free to add a pull request if I haven't gotten to this by the time you get some time to look into it. Thanks for the suggestion!