Implement Mindesteinkommen for WoG.

[MImmesberger]

Closes #592

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.77%. Comparing base (f855be4) to head (b38f14b).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #772 +/- ## ========================================== + Coverage 89.63% 89.77% +0.14% ========================================== Files 53 53 Lines 3771 3804 +33 ========================================== + Hits 3380 3415 +35 + Misses 391 389 -2 ```

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[MImmesberger]

On 3 occasions, regression tests fail after switching to Lohnsteuer. Still investigating.

[hmgaudecker]

That may well be! Lohnsteuer should be the correct thing to use, so unless these are "official" test cases, it might not be a big source of worries.

[MImmesberger]

Not sure how official they are. Jakob didn't create test cases that reach back to 2009, right?

[hmgaudecker]

I didn't think he ever did any for Wohngeld. Unless they state a source, you may assume they are regression tests from a time where we did not have Lohnsteuer.

[MImmesberger]

FYI: I removed two input variables because we can compute them now via our usual machinery.

[MImmesberger]

I'll start with a new PR given the requested changes back to hat_kinder and of Lohnsteuer back to Einkommensteuer. Will cherry-pick the commits here.