SMM11 — Introduce Unit of Measure as a class.

Currently DimensionalMeasure has Unit of measure as an attribute with type string. Unit needs to be treated as a first class object and not just indicated by a name. A class need to be introduced for it and the attribute needs to be replaced with an association relating DimensionalMeasure to a Unit. Indication by name leaves open whether “meter”, “Meter” or “metre”. But the object for each would be the same and the case and internationalization would be left to rendering.

Replace the "unit" property (string) of DimensionalMeasure, by a "unit" association from DimensionalMeasure to UnitOfMeasure. This will greatly improve re-use and consistency of units, across DimensionalMeasures. Unit conversion can be left to rendering.