Move spherical harmonic basis keyvalue?

[Lestropie]

Currently the spherical harmonic basis used is specified in the model JSON file, rather than the ODF JSON file. With inheritance this makes sense only if all of the model derivatives are SH ODFs. Likely a better solution would be for the individual ODFs to have within their sidecar files the SH convention used. That would prevent that information from being inherited by files to which it is not applicable, and would mean that any ODF image - metadata pair can be reasonably interpreted without necessitating reference back to the model JSON.

[Lestropie]

Intrinsically addressed by #92, as there is no model-wise JSON file, and "OrientationEncoding" is defined individually for each data file.