Improve SubmodelElementList implementation

[s-heppner]

Currently the implementation of SubmodelElementList can only be described as not ideal.
We set a random id_short, even though the specification forbids it, due to the fact that we need an identifying attribute in our Namespace.
Of course, when serializing we remove the id_short, however it would be better if we didn't need it in the first place.

We should research how other AAS-SDKs solved that issue and improve our implementation.