model: Make HasKind.kind immutable
This is a proposal to make handling of objects and enforcing of constraints more safe. Idea: A template-object should never be converted to an instance-object (without explicit creation of a new object) and vice versa. This would resolve the issue with constraint AASd-008 (see #6 (closed)).
We could also consider making other, more specific attributes immutable.
Implementation is possible using a property and a hidden attribute (