Split IO into separate package

In the scverse core team the consensus was reached that IO should not be part of the analysis packages (e.g. scanpy, scirpy, muon), but rather in an independent package with minimal dependencies and have the analysis packages depend on it. The hope is that this leads to a wider adoption of scverse datastructures, since the "dependency cost" of depending on a lightweight IO packages is lower than depending on an entire framework.
This issue is to track the goal of creating such a package for scirpy.

Name (?)

A couple of ideas

• scirpy-io
• scverse-airr
• airr-io

Scope

• All read_xxx and write_xxx functions in scirpy.io
• AirrCell, to_airr_cells and from_airr_cells functions
• to/from_dandelion (ideally dandelion adapts the scverse datastructure. Otherwise these functions should live in dandelion itself)

Maybe

• merge_airr
• index_chains
• get.airr

The latter two go beyond just storing AIRR data as an awkward array, but implement the scirpy receptor model. But they are likely useful for some other packages. But then again if a method needs this, they could just depend on the full scirpy.

In case of doubt, err on the side of including less in the package, as it could be added later if required.

Start by inventorying the functions listed under the scope in scirpy.io, including the read_xxx and write_xxx functions, AirrCell, and the conversion helpers. Review the optional merge_airr, index_chains, and get.airr items separately, then determine the smallest independent package boundary and candidate name. Done means the scoped IO functionality is separated with minimal dependencies and scirpy can depend on it.