Add TrafficDistribution as catalog element

Describe the feature

When using the new TrafficDistribution for several scenarios, the TrafficDistribution has to be defined in the scenario over and over again and cannot be reused.

Describe the solution you would like

Add a catalog element of type TrafficDistribution.

Describe alternatives you have considered

Copy&paste of the TrafficDistribution for every scenario is bad style, error-prone and hard to maintain.

Describe the backwards compatibility

Backward compatible.

Additional context

The interface "CatalogElement" needs to be implemented by the TrafficDistribution class.
Also in class "CatalogLocations" a "TrafficDistributionLocation" must be added.
The class "Catalog" must be extended to contain "TrafficDistribution" as well. All usages of the TrafficDistribution class must be replaced by an XOR for a TrafficDistribution or a CatalogReference (is backward compatible).
Chapter 9.4 of the User Guide (Catalogs) must be adjusted as well to contain the new catalog element type.