Proposed 3.0 model change to Annotation

[goneall]

Current SPDX 3.0 model does not support the use case where an annotation is created which refers to an Element in a separate Document. This is due to the Annotation being a property of the element - so if you want to create a separate annotation for an element, you need to modify the element to add the annotation.

This can be solved by making the following changes:

• Remove the annotation property from Element
• Add a property to Annotation which references the Element it annotates
• Add a property Document which includes all Annotations defined within that document [this would be convenient for serializations, but not essential for the domain model]

The proposed changes are similar to how the Relationship is currently modeled.

Positives of the proposed change:

• Annotations can be created independent of the Element being annotated - useful for reviews, corrections, notes being added after an Element is originally created
• Matches the way tag/value serializes annotations in SPDX 2.2

Negatives of the proposed change:

• Harder to find annotations associated with an Element. If the Annotations are in the same Document, you need to re-associate the Annotations to the Element. If the Annotations are in a separate document, correlating the Annotation to the Element is even more difficult.
• Incompatible with the current SPDX 2.2 model which has Annotations as a property of Element