implement constraint AASd-120

[jkhsjdhjs]

generate unique id_shorts for SubmodelElementList-children

Constraint AASd-120 requires direct children of a SubmodelElementList
to have id_short=None. On the contrary, SubmodelElementList must be a
Namespace, since children of Lists must still be referable via
References, and also must be allowed to reference their parent, which
is expected to be a Namespace.
Since id_short=None must hold for all direct children, they lack
a unique identifying attribute, that can be used to refer to an
item. However, this is required for a Namespace.

Thus, we had two options for implementing this:

• Refactor a lot of the model.base module such that SubmodelElementLists
  are considered Namespaces
• Generate a unique id_short for every direct children of a
  SubmodelElementList whenever it is added.

Since the first alternative would require a distinction for
SubmodelElementList in all places where a Namespace is used, we
decided on the second alternative.

This commit implements the generation of unique id_shorts via the
item_id_set_hook, that was recently added to NamespaceSet and
OrderedNamespaceSet. It is called for every added SubmodelElement.
Furthermore, the item_id_del_hook is called for every removed
SubmodelElement and used to remove the generated id_short again.

This aside, the examples and unit tests are also adjusted such that the
id_short is removed for all direct children of SubmodelElementList.
Furthermore, a test for AASd-120 is added.
The AASDataChecker is adjusted to skip the comparison of id_short for
direct children of SubmodelElementList, since these are generated and
thus never the same now.
For the same reason, the XML/JSON serialisation is adjusted to skip
serialising the id_short if direct children of a SubmodelElementList.

examples.data._helper: disable comparison of unordered SubmodelElementList

Since direct children of SubmodelElementList don't have an identifying
attribute anymore (AASd-120), they cannot be compared because it is
impossible to know which SubmodelElement should be compared against
which other element. Maybe this can be implemented again in the future,
when hashing is implemented for all SubmodelElements, but for now we
raise a NotImplementedError.

A test-case for this behavior is added and order_relevant is set to
true in all example files.

test: update compliance tool test-files

This commit applies the following changes to all test-files:

• The id_short of direct children of a SubmodelElementList is removed
• SubmodelElementList.order_relevant is set to true for all
  SubmodelElementLists

This PR depends on #132

[s-heppner]

Wow. Thank you for conquering this monstrosity!

It looks good to me, so I can rebase it when you're ready.

[jkhsjdhjs]

Yep 👍