Physical units

[ivirshup]

Now that coordinate spaces and transforms are defined, I think we need to start thinking about handling physical units.

How do we plan to do this?

[LucaMarconato]

Implemented in the https://github.com/scverse/spatialdata/tree/feature/transform_and_coord_spaces/spatialdata branch (not ready for pull request, I will ask for review when it's ready).

[ivirshup]

I think we should only close things once the feature is merged

[melonora]

@LucaMarconato I think we seriously need to discuss this now as with napari 0.5.0 units can now be passed to the viewer.