Support min/max

[quentinblampey]

Hi @flying-sheep,
Thanks for this package, it looks very useful to support many types in adata.X!

I need some similar functionalities for .min(...) and .max(...), and, as far as I understand, it's not implemented yet in this package?
If so, I can contribute and open a PR if you're interested

[flying-sheep]

For sure, this would be great! Thank you!

[quentinblampey]

Great, I'll work on that in the next few weeks or so!