WIP: Data scaling modelling

This is a proposal, made with @nicholst, to model data scaling.

Typically, for a first-level model the attributes of the `image Data` entity would be something like:
`prov:type = nidm:imageData
prov:type = collection
prov:label = "Image Data"
nidm:grandMeanScaling = True
nidm:targetBrainIntensity = 100
nidm:measuredMedianBrainIntensity = 115.3`

and for a second-level we would have something like:
`prov:type = nidm:imageData
prov:type = collection
prov:label = "Image Data"
nidm:grandMeanScaling = False`