Dependencies between model versions

As models are built on models, it should be obvious that the situation may arise that a model requires another model that has at least a particular version - let’s say the first version that introduces a type that the dependent model requires.

Conversely, when a model changes non-monotonically, authors may have to adapt dependent models to the new situation.

We would like to adminstrate these dependencies and to have support in the PDR that prevents problems.