Implement SWNE embedding

[adkinsrs]

• [ ] Additional function parameters / changed functionality / changed defaults?
• [x] New analysis tool: A simple analysis tool you have been using and are missing in sc.tools?
• [x] New plotting function: A kind of plot you would like to seein sc.pl?
• [ ] External tools: Do you know an existing package that should go into sc.external.*?
• [ ] Other?

For gEAR (Gene Expression Analysis Resource), we have had a few users request Similarity Weighted Nonnegative Embedding (SWNE) plots instead of tSNE or UMAP plots. Having this embedding option (scanpy.tl and scanpy.pl) would allow use to expand functionality in several of our tools without having to leave the scanpy environment in order to provide them.

Repo: https://github.com/yanwu2014/swne Paper: https://www.sciencedirect.com/science/article/pii/S240547121830440X?via%3Dihub ...