Integrate Milopy

[Zethson]

We want to integrate https://github.com/emdann/milopy into pertpy with an updated version

Tasks (roughly in order) are:

• Get comfortable with milopy -> Install it and run some example notebooks.
• Use the current code from this branch https://github.com/emdann/milopy/tree/mudata
• Create a new _milopy.py file.
• Add a Milopy class to the file + add all corresponding functions that are not plots.
• Ensure that the style of the code adhere's to pertpy's style -> comment formatting, black etc etc
• Add all plotting functions to pertpy.plots in a new file as we've done for all other tools + alias the import + fix code style
• Add all tests from the branch and ensure that they are all running + fix code style
• Create a new notebook here https://github.com/theislab/pertpy-tutorials/ based on the Milopy one. Now make it work with pertpy.
• Try to solve the feedback from Refactoring with MuData emdann/milopy#26 -> better MuData support
• parameterize the GLM solver -> edgeR and batchGLM -> wait for @picciama to be ready
• Test that everything works :)