ENH: Improve current implementation of Vorrangprüfung for Wohngeld

[MImmesberger]

This issue is based on #710.

Wohngeld has priority over ALG2 if the recipients can cover their needs according to SGB II when receiving Wohngeld.

The priority check that we have currently implemented follows this logic:

1. Calculate Wohngeld on the Bedarfsgemeinschaft level.
2. Check whether the Bedarfsgemeinschaft can cover its own needs (Regelbedarf) with Wohngeld. If not, the Bedarfsgemeinschaft is eligible for ALG2.
3. Compute Wohngeld again for all individuals in the household that can cover their own needs with Wohngeld. This is the final Wohngeld amount that is paid out to the wohngeldrechtlicher Teilhaushalt.

Note: Because Wohngeld is nonlinear in the number of people in the wohngeldrechtlicher Teilhaushalt, there may be some individuals that pass the priority check, but cannot cover their needs with the Wohngeld calculated in point 3. In this sense, this implementation is an approximation of the actual Wohngeld.