Inconsistent terminology for intensity units of lights in draft 3D api and example manifest

[vincentmarchetti]

The intensity of a light https://github.com/IIIF/3d/blob/main/temp-draft-4.md#lights is (or can be?) defined as a resource of type "Value" with a "unit" property. The code example in the draft 3d API ( https://github.com/IIIF/3d/blob/main/temp-draft-4.md#lights ) shows a value of the "unit" property as "relativeUnit". However, the example manifest https://github.com/IIIF/3d/blob/main/manifests/3_lights/ambient_green_light.json the unit name of "relative" is used.

I propose that as the 3D API is further specified the term "relative" be maintained as referring to a relative intensity scale, with 0.0 meaning the light is off, and 1.0 denoting the maximum intensity supported by the rendering software and display technology.