HO04 and c# New Extension

- HO: Added merged customers (Clear obsolete code in RestService
classes)
- c#:  Improved presentation of 'total prescription price' in
Prescription/List
- c#: Fixed exception handling c# prescription add drug (no pzn given;
drug cannot be found; already exists; age check fails) -> New
understanding: throw new argumentException in
DrugService/PrescriptionService directlc connected to AddDrug_Command
catch (ArgumentException ex)