ENH: Improve implementation of Vorsorgeaufwand

[MImmesberger]

Is your feature request related to a problem?

Currently, the Vorsorgeaufwand module is not up to date with our current machinery. In particular, time conversions happen manually and the naming of functions is unclear to a point where the DAG is not comprehensible from looking at the function definitions.

For example, we define:

@dates_active(
    start="2005-01-01", end="2019-12-31", change_name="vorsorgeaufw_y_sn_bis_2004"
)
def _vorsorgeaufw_y_sn_bis_2004(

Describe the solution you would like to see

Make time conversions outside of functions. Improve function names.