adding AutoGeneS as an external package to scverse

[lila167]

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

I would like to add AutoGeneS (https://github.com/theislab/AutoGeneS) as an external package. It's a bulk deconvolution tool that is based on anndata and scanpy.