ModelReference type deserialization

[jkhsjdhjs]

Currently, the type of a ModelReference is set as Referable for many attributes. Maybe it should be determined by checking the last key instead? The last key is currently only used to validate the type of a ModelReference, but not to determine it.

[s-heppner]

I moved this text from #367:

In JSON, the, _construct_model_reference is very often called with the generic model.Referable as type_, instead of one of the more concret subclasses (e.g. model.Property).
This means, a model.ModelReference then has just the generic model.Referable as type attribute, leading to problems when trying to analyze the Reference further in the code.

I suggest to improve the calls of this deserialization function so that the actual type of the model object that the ModelReference points to is written to the ModelReference.type.

This issue was first noted in #337, where a hotfix is implemented, by inferering the type the ModelReference points to by using the last_key_type of the last Key in the References Keys.
Once this issue is solved, we should remove the then unnecessary hotfix from the _construct_model_reference method, that is marked with # TODO.