add a parameter`n_jobs` in create_knn_graph()

[hanguojun007]

Hi, I hope this parameter can add in this function, or maybe I should learn some Class of python.~ and when i use the distence of about 100,000 cells, it is very slow.

[adamgayoso]

This is now addressed with approximate nearest neighbors

https://github.com/YosefLab/Hotspot/pull/14