Improve and expose axes interface

[gtca]

Changes that are part of this PR:

Improved update

• Update ._attrhash functionality
• Store ._varhash and ._obshash
• Update .update()
• Optimise for when index hasn't changed
• Allow to update_attr() when new modalities are added

Axes interface

• Add tests for axis=1
• Add a notebook explaining axes in MuData
• Improve axis=-1 interface
• Add tests for axis=-1

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