[V3.0] Implement missing constraint logic for AASd-117.

[s-heppner]

This implements the missing invariant for AASd-117 namely, that any Referable that is not direct child of a Submodel_element_list must have its ID_short set. This fixes #253.

Since it is impossible to access an object's parent, we need to implement it in a creative way. Therefore, we add the invariant to every Referable object that it can apply to:

• Asset_administration_shell: An AAS-object can never be a child of a Submodel_element_list

• Submodel: Similarly, a Submodel-object is never a child of a Submodel_element_list. Additionaly, every child of a Submodel is evidentally not a child of a Submodel_element_list.

• Submodel_element_collection: Every child of a SEC is never a child of a Submodel_element_list. Careful, though, a Submodel_element_collection may be a child of a Submodel_element_list though. Therefore, we cannot check for its own ID_short to be set.

• Operation_variable: has a value, that is not child of a Submodel_element_list, therefore check that its ID_short is not None

Note that the logic already existed for most objects, but there was no reference to the constraint

[mristin]

@s-heppner thanks! We also need to add assertion test for that logic to make sure we didn't miss anything.

[s-heppner]

Someone please check the docstring of ID_short_type. Something was very wrong with the docstring references and I got a ton of errors

[mristin]

Superseded by #256.