Better compatibility with BiocNeighbors output

[jlmelville]

A workaround is to build the neighbor graph outside of umap2 and pass the results to the nn_method parameter.

Thank you for the advice. I now use findKNN() from BiocNeighbors with their default KmknnParam to find the exact knn and notice that their naming convention is list(index, distance), not list(idx, dist). Still, I'm glad the documentation states it clearly so it's not bad at all.

Originally posted by @hsuknowledge in #139

Where nn_method allows for nearest neighbor graph list input, index and distance should be allowed.