SMM11 — Clarify that Observation.requestedMeasures is of type SmmElement and not AbstractMeasureElement

It is currently the case that Observation.requestedMeasures is of type SmmElement. Constraints have been added to make sure that one does not pass a e.g. SmmModel instance as a requestedMeasure but only instances of type Measure, MeasureCategory and CategoryRelationship.

My suggestion is to change the type from SmmElement to AbstractMeasureElement. We would then be able to measure the following:
MeasureCategory - e.g. Measures that fall in the same category
Scope - e.g. Measures that use the define the same scope
Characteristic - e.g. Measures that are characterized by a certain characteristic
Operation - e.g. Measures that use a certain operation
OCLOperation - e.g. Measures that use a certain OCLOperation
Measure

This would make the metamodel cleaner by having lessing constraints and making it easier to understand. Could it be that this was caused because of the fact that there is no section about AbstractMeasureElement?

Change the type of requestedMeasures to be AbstractMeasureElement