Update tutorial to demonstrate image reading

[dstansby]

This:

• Adds a bit of sample data we can use with the tutorial (and perhaps tests later?)
• Demonstrates how to read this data in the tutorial, and shows a plot
• Modifies Image a bit to hide away pydantic_zarr from the public API. Instead of sub-classing this just uses GroupSpec to do validation and then store the attributes, but these are provided to the user in the form of custom properties.

I'm not 100% sure this is the best way to go in terms of what our "top level" object is and looks like, but I'm wary about sub-classing from another library as we'll lose the flexibility of doing what we want. Very open to other suggestions/ideas though, ideally with modifications to the tutorial to show how they would work as public API.

[dstansby]

This is a bit of a hack for testing

[dstansby]

I need to fix the tutorial, but would be good to hear your thoughts on this @d-v-b, in particular:

Modifies Image a bit to hide away pydantic_zarr from the public API. Instead of sub-classing this just uses GroupSpec to do validation and then store the attributes, but these are provided to the user in the form of custom properties.

[dstansby]

There's a better version of this at #64