model.base: Protect NamespaceSet attributes from being replaced

[zrgt]

Attributes like Submodel.submodel_element, Qualifiable.qualifier or
HasExtension.extension hold a NamespaceSet, while the corresponding
__init__ parameters accept any Iterable. Assigning an iterable to
such an attribute after initialization silently replaced the
NamespaceSet with that plain object. The old NamespaceSet remained
registered in namespace_element_sets, so its objects stayed reachable
via get_referable() and friends while being invisible in the
attribute, uniqueness constraints were no longer checked and the
parent of the new objects was never set. The resulting errors, such as
AttributeError: 'list' object has no attribute 'update_nss_from' when
iterating over an ObjectStore, gave no hint about the actual cause.

Namespace.__setattr__ now redirects such assignments to the content of
the existing NamespaceSet, which is the behavior a user would expect
from the type of the __init__ parameters:

submodel.submodel_element = [my_property]  # replaces the content of the set
aas.extension = []                         # clears the set

As discussed in the issue, this is done centrally in Namespace instead
of via a property per attribute. That way it needs no repetition for
every current and future NamespaceSet attribute and also covers
NamespaceSet attributes of classes defined outside of this repository.
The existing SubmodelElementList.value setter keeps working unchanged.

The replacement is atomic: if one of the new objects violates a
constraint, the previous content of the set is restored, so a failed
assignment leaves the object untouched.

Fixes #65