Adds tests to test reading of legacy files

[votti]

This adds a mechanism to test for backwards compatiblity of reading mudata files created with older mudata versions.

This should enable automated detection of breaking of backwards compatiblity in the future and make it easier to add tests if people report backwards incompatiblities.

This test is modeled after the anndata tests for backwards compatiblity: https://github.com/scverse/anndata/tree/main/tests/data/archives
https://github.com/scverse/anndata/blob/main/tests/test_io_backwards_compat.py

Considerations:

• This uses uv scritpt header for create_testfiles.py to improve reproduciblity. It may be also feasible to create mudata.h5mu files by running cretate_testfiles.py using github actions before running the tests. Given how small the mudata.h5mu files are, the overhead of on-the-fly creating the python environments using uv would be bigger.
• Alternatively it could be considered managing the archives of older testfiles in an s3 bucket or other storage. Again as the files are only kbs, this may be to much of an overhead.
• Extensibility: currently the test only checks if there exists mod1 and mod2 with the correct dimensions in this files. Potentially additional data could be added to make these test more comprehensive.
• One create_testfiles.py per version: while it may be possible to have one centralised create_testfiles file/class, It seems more flexible to have one file per version, as this would also enable to make version specific changes/adaptations to the script more easily.