refactor(au): support multiple AX_KommunalesGebiet per AX_Gemeinde

[florianesser]

Allow for multiple AX_KommunalesGebiet instances to exist that have the same gemeindekennzeichen. With this patch, the geometries of these AX_KommunalesGebiet instances will be aggregated instead of only one being selected randomly.

SVC-890

[JohannaOtt]

The definition of AX_KommunalesGebiet according to the ATKIS OAK is: 'Kommunales Gebiet' ist ein Teil der Erdoberfläche, der von einer festgelegten Grenzlinie umschlossen ist und den politischen Einflussbereich einer Kommune repräsentiert (z.B. Stadt-, Landgemeinde, gemeindefreies Gebiet).
I read "den politischen Einflussbereich" in a way that there is only one per Gemeinde (not "einen politischen Einflussbereich"). So before reviewing it from a technical point of view, I would like to discuss if that case is allowed at all or if it should rather lead to an error being thrown.
@florianesser Do you read the OAK differently?

If the change is kept after reviewing the definitions etc.: Would it make sense to align it with the code used in the kommunalesGebiet alignment (creating a union)?
If the change is not kept the union should probably be removed from the kommunalesGebiet alignment.

[florianesser]

Do you read the OAK differently?

@JohannaOtt I would not interpret "den politischen Einfluss" to mean that a Gemeinde cannot have exclaves. There are real-life examples that a Gemeinde can have multiple kommunale Gebiete in the Basis-DLM data of Hessen. Without this patch, the output of the transformation is incomplete.

Would it make sense to align it with the code used in the kommunalesGebiet alignment (creating a union)?

Yes, that might make sense, I'll check that.