Revise mandatory parameters for updates

[f-sander]

When updating an SLI, there must a property unit with a value present in the supplied json. However, that doesn't seem to be necessary, which leads to the insertion of dummy unit-values without meaning. The API should reflect "mandatoryness" of fields and parameters.

To expand this, it would be really nice, if update operations accepted a subset of the definition and updated only those fields present in the supplied json. This could be done by first reading the stored definition, overriding the fields according to the supplied json and then checking its validity before writing the updated definition back.