ENH: Use ALG 2 Kindersofortzuschlag for KiZ

[MImmesberger]

Is your feature request related to a problem?

There's a period of 6 months where the Sofortzuschlag is relevant for KiZ explicitly. We have the same parameter in the ALG2 and the KiZ parameters, in the latter we just set it to 0 once its not relevant for KiZ anymore.

Describe the solution you would like to see

We should use the ALG2 parameter and handle this in maximales_nettoeinkommen_m_bg explicitly (split up into two and use start_date/end_date).

[MImmesberger]

Closed via #912