Identification of all elements in the model not straight-forward

[linuswagner]

Getting an overview of all components in the system is difficult.
Intuitively, one would imagine, that all relations contain elements from the declarations relation.

However, it turns out that this must not be true: Some elements in the callgraph can be missing in the declarations, the same is true for some elements in the methodOverrides. Namespace elements are also not declared and can only be found in the containment relation.

Not knowing this, I was tempted to think of declarations as a complete list (within the correctness of the model) and did not look for other elements. It would be good to either have this documented (relates to #64) or to provide functionality that allows to quickly get all elements from the M3 model (i.e. going over all relations and get the domain/carrier for all logical names), so it becomes explicit that there is no collection that by default has all elements from the model.

[jurgenvinju]

Thanks Linus!