make `Namespace` iterable

@zrgt suggested on Zulip, that classes like Submodel and SubmodelElementCollection should be iterable.

@mhthies suggested to make Namespaces iterable with an implementation similar to the following:

class Namespace:
    […]

    def __iter__(self):
        return itertools.chain.from_iterable(self.namespace_element_sets)

This would make all classes which inherit from Namespace iterable over their contained NamespaceSets, namely AssetAdministrationShell, Submodel, SubmodelElementCollection, AnnotatedRelationshipElement and Entity.