REFAC: rewrite to_representation() in agg_serializer.py

Rewrite the general to_representation() function in utils/agg_serializer.py.

• remove duplicating declaration of properties dictionary if direction is given ✔ [alread during testing for #31 (closed)]
• remove duplicating declaration of properties dictionary in general (recently it is defined twice) ✔ [solved with 1b6f428b]
• check if some branches can be unified. Reason for old structure: there was the *by_direction' parameter. But this is removed with #31 (closed).Maybe there is a better solution. Following combinations are possible: single value, with direction, multiple values + single direction, + multiple directions, ...?