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`.

…ntList`

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.

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
  `SubmodelElementList`s

 Set to Reference instead of ExternalReference according to specs

This commit extends eclipse-basyx@bffb075.

…ated_id_shorts_aasd_120' into fix/id_short

As default value of Submodel.id_short, ConceptDescription.id_short, AAS.id_short is set to None, test files had to be updated.

As default value of Submodel.id_short, ConceptDescription.id_short, AAS.id_short is set to None, test files had to be updated.

- Update IEC61360_DATA_TYPES from the Spec
- Add checking of value_type string: 1<len(value_type)<2000
- Some Refactoring
- Fix typehint of DataSpecification according to the spec
- Simplify Value and ValueReferencePair, as value is just a string acc. to spec
- Fix value_format, as it not one of xsd types, but also a string (e.g. "X..17" or "M" or "NR2..3.3"), which describes how the value should look like. Look in IEC61360-1 for further info