code: public String getGroupedValue() {
          StringBuilder sb = new StringBuilder();
          sb.append(getRegisternummer()).append(Constants.DOT);
          sb.append(getAccountType()).append(Constants.DOT);
          sb.append(getPartAfterAccountType());
          return sb.toString();
      }
docs: Returns the Kontonummer as a String, formatted with '.''s separating the
  Registernummer, AccountType and end part.
  
  @return The formatted Kontonummer