ENH: Handling of dates

[hmgaudecker]

We still have an ad-hoc evaluationsjahr in the taxes & transfers DAG. Here is the discussion from #960
:

graph TD;
    date_str --> date;
    evaluation_date_str --> evaluation_date;
    date --> evaluation_date;
    date --> policy_date;
    policy_date_str --> policy_date;

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All these would live in the interface DAG, the evaluation date would also go into the taxes & transfers DAG. The str variables would be inputs, the others functions. Raise an error if both date and either specific date are passed.

[MImmesberger]

Done in #985