Issue #2-9: Mandatory Relationship between Value Domain and Measurement Unit

(Feedback from Australia) 

This change was imported from CSPA LIM, as part of aligning GSIM 1.5 with the latter, rather than having a specific issue number in the development of GSIM 1.5.

A question was raised during ABS review, however, about the mandatory relationship to Measurement Unit given Value Domains may be Categorical/Enumerated.

It would be possible to define an arbitrary Measurement Unit (eg "Category") for categorical Value Domains but this adds no value given the fact the Value Domain itself is of sub-type Enumerated Value Domain.

A possible solution might be to move the relationship more specifically to Described Value Domain. Even there a Described Value Domain that contains a text string (or arguably a simple date rather than a time duration) does not have a meaningful Measurement Unit.

The ABS implementation addresses this by further sub-typing Described Value Domains and making the equivalent relationship mandatory for Numeric Value Domains.

A simpler solution for GSIM might be the approach used in the current version of ISO 11179 Part 3. ISO 11179 has Dimensionality (GSIM:MeasurementType) as 0..1 for Conceptual Domain and Unit of Measure (GSIM:MeasurmentUnit) as 0..1 for Value Domain.