Investigate whether its worth supporting collecting missing references for relationships and models

[nick4598]

In a recent PR to rework the partially committed element references tracking: #206

Vilius mentioned this about relationships and models in response to my question "What is the thought process behind removing the models and relationships?":

The transformer did not handle those before as well. The collectUnmappedReferences was only called from 3 places: onExportElement, onExportElementUniqueAspect, onExportElementMultiAspects.

References to Models will be resolved the same way as references to Elements, because transformer will check for existence of a Model through its modeled element which will always have the same Id.
Not sure if Models themselves can have any references to Elements other than their modeled element that could be dangling, but the transformer did not do anything about those before this update as well.

As for Relationships, exportRelationships is called after all elements and aspects will already have been exported, so in theory they should never have dangling references. Also, a relationship would have to have a reference to a 3rd element other than its target and source elements for such case to occur.

Questions to answer:

• Can a relationship have a reference to an element that isnt inherently part of its relationship? (i.e. not source and target IDs)
• Can models have references to elements other than their modeled element:

I assume this is relatively low priority since its not currently supported by the transformer.