Bring Physical Dimensions into core Presentation 4

[tomcrane]

IIIF has an extremely powerful but little-used feature available as an external service:
https://iiif.io/api/annex/services/#physical-dimensions

While not widely implemented (usually through lack of data), it is very powerful when used, and is going to be much more important in 3D, where this kind of data usually is available for typical IIIF use cases.

The 3D work requires introducing a class to carry values with the units the values are expressed in. Another use of such a class could be the value of a physicalDimensions property, available on Canvas and Scene:

As the value of a physicalDimensions property on a Canvas, the following states that each integer unit of the canvas coordinate space represents 0.00025m

{ 
   "type": "Value", 
   "value": 0.00025,
   "unit": "m"
}

(NB we might think of better names for the class and its properties)