allow `backup_url` for read functions in MuData

[AnnaChristina]

For convenient file loading from back-up URLs would be great to have a similar argument as in scanpy.read that allows reading files from backup URLs which retrieves the file from an URL if not present on disk.

[grst]

I would prefer to see pooch being used in such cases instead.

Here's an example how it can be used in tutorials: https://scverse-tutorials.readthedocs.io/en/latest/notebooks/basic-scrna-tutorial.html

[AnnaChristina]

thanks! closing this, was mainly interested in using this for the book but will keep things then consistent with your recommendation.