ENH: Get rid of input variable `kind`

[hmgaudecker]

Is your feature request related to a problem?

With the structure from #694, the input variable really stops making sense. But the PR is bloated already, so not implementing there.

Furthermore, it is not clear in which sense it is a child. Bundeskindergeldgesetz? Under 18? ... ? Probably mostly follows old IZA $\Psi$ MOD / SOEP conventions.

Describe the solution you would like to see

Just calculate endogenously (age, part of household, parent ids, ...), potentially depending on the context.

[MImmesberger]

For ALG2 Regelbedarf (arbeitsl_geld_2_regelsatz_m_bg in particular), "erwachsen" refers to the age threshold of 18 years. So there it doesn't relate to BKGG.

[hmgaudecker]

Cool, then the check should just be on age there (it might be the case already).

[MImmesberger]

Reminder to keep this comment in mind when redesigning the kind column (if this is not solved via #708)