Todo: Write modification proposal for the obsoleten of waehrungscode

[hf-kklein]

In the Rechnungsposition: If you try to validate this sentence:

Das Ergebnis der Multiplikation aus einzelpreis * positionsMenge * (Faktor aus zeitbezogeneMenge). Z.B. 12,60€ * 120 kW * 3/12 (für 3 Monate).

There is the implicit assertion, that Preis.einheit which is a Waehrungseinheit is EUR
because the Betrag uses waehrungscode and the only overlap of both Enums is EUR.