BUG: Several bugs in Grundsicherung im Alter

[MImmesberger]

Bug description

1. The wealth exemption differs for children depending on whether the individuals in the Einstandsgemeinschaft have a Kindergeld claim. Source
2. The wealth exemption is 10,000 € per individuals since 2023. (done in Vermögensfreibetrag for Grundsicherung im Alter outdated (should be 10,000 EUR since 2023) #1150)
3. As discussed in Remove tax unit groupings. #694, it's not clear whether the ALG2 Regelbedarf on bg level should be used in grunds_im_alter_m_eg. (see Gemischte Bedarfsgemeinschaften #1164)
4. Its unclear how capital income was handled before 2016. Starting from 2016, § 43 SGB XII Abs. 2 is relevant.

[MImmesberger]

A related issue is #703