Rethinking the image API, sketch

[matthew-brett]

We're nearly there with this - we just need to implement something like a to_nifti function, maybe available via the xi accessor, that:

• Checks the orientation of the image data, and transposes as necessary.
• Resolves the metadata to something BIDS-compatible - but I think this is already true.
• Returns a dictionary and a Nifti image.

[matthew-brett]

Also note https://github.com/matthew-brett/xibabel/blob/main/experiments/basic_xibabel.Rmd

[matthew-brett]

Thinking about keeping track of the affine here : https://github.com/pydata/xarray/discussions/8897

[matthew-brett]

This all now implemented in main, closing here.