Move optional dependencies

[jl-wynen]

The individual packages have 'test' and 'docs' optional dependencies. We originally added those for our dependency locking setup. But that is no longer needed. So I think we should move those to dependency groups. That way, they do not become part of the deployed packages.

Question: can pixi handle dependency groups from sub packages?

[MridulS]

Yes it should - https://pixi.prefix.dev/latest/python/pyproject_toml/#dependency-groups - it can read them.