Arbitrary dimensional slicing

[eroell]

Is your feature request related to a problem? Please describe.
Classes that subclass AnnData can also profit from the internal organization from MuData.

Describe the solution you'd like
One cool thing to have would be MuData being slicable for more than 2 Dimensions, and passing this on to the stored objects, who should handle it themselves.

If the objects cannot handle the slicing dimensionality (e.g. AnnData right now), this would still fail gracefully.

Describe alternatives you've considered
Subclassing MuData; which we consider to do as well. This change in MuData would make it more flexible in this point, without impairing its functionality.

Additional context
Thanks for the discussion earlier @gtca !

[eroell]

PR to be opened by @eroell