[3.0, 3.1] Make qualifier constraint implementation specific

[empwilli]

The current implementation of constraint AAS-129 is incomplete: it only checks
the constraint for the direct children of Submodel, whereas the constraint
requires that its condition is checked for all SubmodelElements in the whole
Submodel recursively. This, however, can currently not be expressed in the meta
model description. Hence, we extract the constraint into a separate method and
annotate it as implementation specific.

This addresses #374.

[mristin]

Add a transpilable verification function so that we have the hard parts dealt with outside of the snippet:

    not (self.submodel_elements is not None)
    or (
        not (self.kind_or_default() != Modelling_kind.Template)
        or (
            all(
                not (submodel_element.qualifiers is not None)
                or all(
                    qualifier.kind_or_default() != Qualifier_kind.Template_qualifier
                    for qualifier in submodel_element.qualifiers
                )
                for submodel_element in self.submodel_elements
            )
        )
    )

Add a comment that the two methods must be in sync -- include an assertion in tests/ that the code is exactly the same for the two methods (one for submodel element list and the other for submodel element collection).