BUG: Split up Mietkosten across different Bedarfsgemeinschaften

[MImmesberger]

Bug description

To compute the Bürgergeld Regelsatz for households with multiple Bedarfsgemeinschaften, we need to split up Mietkosten (warm/kalt) as they are defined on the household, not Bedarfsgemeinschaften level.

Proposed implementation

Mietkosten are split up according to the relative Bedarf of the Bedarfsgemeinschaft in the ExMin-Bericht (don't have a source for this).

Alternative solution

Mietkosten are split up according to the relative size of the Bedarfsgemeinschaft (e.g. if 60% of people in the household are part of the BG, 60% of Miete are borne by the BG). This is implemented by #694 but seems to be wrong.

[MImmesberger]

This should also be relevant for #710