[FEATURE] Allow for single Submodels to be shown independently

[aaronzi]

Is your feature request related to a problem? Please describe.
When a ReferenceElement references a Submodel, this submodel could be part of multiple AAS. Currently the UI will look in the registry and takes the first found element.

Describe the solution you'd like
It should be possible to visualize just a Submodel without having to specify an AAS (e.g. via the the aas query parameter in the URL in the address bar). Therefore the AASTReeview and the SubmodelList should also be able to just visualize one Submodel.

Affected Components

• ReferenceElements
• (Annotated)RelationshipElements
• Entities
• SubmodelElementHandling
• AASTreeview + SubmodelList

Related Issues
#67 is related and could be handled in the same PR