Add content.quantity to ArtifactAssessment (pre-applied)

The ArtifactAssessment Resource contains type 0..1 CodeableConcept and classifier 0..* CodeableConcept elements within content BackboneElement to represent rating or classifying content. In the EBM IG we have specific needs to support quantitative ratings and inventing CodeSystem and ValueSet based methods to do it seems overly complicated. Please add ArtifactAssessment.content.quantity 0..1 Quantity (short: Quantitative rating, definition: A quantitative rating of the artifact)