3D model physical dimensions

[gjcope]

https://preview.iiif.io/api/prezi-4/presentation/4.0/#conveying-physical-dimensions describes using spatialScale to assign real-world units to a canvas or scene. For 3D, this works well when needing to compare scale between scenes but could be useful if applicable to the 'model' object as well when dealing with models captured (or created) in different units.

Transforms could be used in the manifest to bring these models to a common scale, but this lacks transparency for the end user to know the unit of the underlying model data.