Implement scverse datastucture

[grst]

Implementing the changes suggested in #327.

Close #327 Close #184

Requires

• pre-release version of awkward array
• Anndata installed from the val_shape branch

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TODO

• [ ] store index pointing to primary and secondary chains somehwere
• [ ] update all tests and test data
• [ ] update example data in scirpy.datasets
• [ ] phase out support for the scirpy <0.7 data structure. The additional effort maintaining this is not worth it.
• [ ] update the _check_upgrade_schema() wrappers to detect the 0.7 < x <= 0.11 data structure.
• [ ] conversion from old schema to new datastructure
• [ ] instructions how to use the new AIRR/awkward array anndata with muon
• [ ] Go through tutorials
• [ ] Go through docstrings
• [ ] keep public functions that were not deprecated before and add stub functions
• [ ] do i still need has_ir and multichain columns?