Tidy up function `unterhaltsvors_m` and fix bug

[ChristianZimpelmann]

• Make the function unterhaltsvors_m nicer and fix a bug introduced in #650.
• Also added new test cases for the (former) bug and the youngest child category (which haven't been tested before)

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.07% :tada:

Comparison is base (6929fb1) 90.53% compared to head (61dff5c) 90.60%. Report is 1 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #652 +/- ## ========================================== + Coverage 90.53% 90.60% +0.07% ========================================== Files 48 48 Lines 3095 3099 +4 ========================================== + Hits 2802 2808 +6 + Misses 293 291 -2 ``` | [Files Changed](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/652?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iza-institute-of-labor-economics) | Coverage Δ | | |---|---|---| | [src/\_gettsim/transfers/unterhaltsvors.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/652?src=pr&el=tree&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=iza-institute-of-labor-economics#diff-c3JjL19nZXR0c2ltL3RyYW5zZmVycy91bnRlcmhhbHRzdm9ycy5weQ==) | `100.00% <100.00%> (+14.28%)` | :arrow_up: |

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