Port sparse matrix support from umap

[tomwhite]

Fixes #6

[lmcinnes]

Thanks for this. You may want to check the 0.4dev branch of umap however, as it has extra distance functions in the sparse case, and a few other potentially useful changes. Alternatively we can get this merged and then worry about the extra stuff from 0.4dev in a later update.

[tomwhite]

I think we can look at 0.4dev separately.

(I just fixed conflicts and rebased.)

[lmcinnes]

Excellent, thank you!

[tomwhite]

@lmcinnes regarding 0.4dev, I just had a quick look and it seems fairly divergent from master. Is the plan to sync it with master - or vice versa?

[lmcinnes]

I potentially need to resync master into it at some point, but eventually it will get merged into master.

[tomwhite]

OK. In that case I think it makes sense to wait until it's merged into master before porting any changes over to pynndescent.