DWI derivatives: Provide diffusivity units

[Lestropie]

Draft pull request as I'm undecided myself, and it may require further modification if we commit to it.

How should the units of any model-derived diffusivity parameter be handled?

1. Compulsory to be specified in JSON
2. Default units in specification, but can be specified in JSON
3. Compulsory units in specification

[codecov]

Codecov Report

Merging #53 (3e43e31) into bep-016 (e485551) will not change coverage.
The diff coverage is n/a.

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[arokem]

I like number 2 the best (with mm^2/sec being the default 😄 )

[Lestropie]

With #92, specification-dictated parameters are no longer there. So there's no special allocation for diffusivity over anything else; identification and interpretation of data is up to a combination of entities & metadata contents. If we want to re-introduce those (which there's a valid argument for), then any possible handling of default units for specific parameters would need to be reconsidered from scratch for the full set of proposed baked-in parameters.