Several small fixes related to groupings

[ChristianZimpelmann]

What problem do you want to solve?

Closes #745

Several small fixes related to groupings.

Leave open for now in case more comes up.

Todo

• [x] simplify use of bruttokaltmiete_m_bg
• [x] fix bug in docstring
• [x] add unterhaltsvors_m to default targets instead of unterhaltsvors_m_hh (let the user decide on which unit to aggregate)
• [x] shorten very long name of test which contains duplicate and is too long for Windows
• [x] Document PR in CHANGES.md.

Note:

• for now, adjusted how Unterhaltsvorschuss is deducted from Wohngeld (see the discussion in #357). However, I am still unclear what is correct here.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 88.88889% with 3 lines in your changes are missing coverage. Please review.

Project coverage is 89.38%. Comparing base (9f04722) to head (c2c6a95).

:exclamation: Current head c2c6a95 differs from pull request most recent head 8cb2ebd. Consider uploading reports for the commit 8cb2ebd to get more accurate results

Files	Patch %	Lines
...ttsim/transfers/arbeitsl_geld_2/arbeitsl_geld_2.py	72.72%	3 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #725 +/- ## ========================================== - Coverage 89.42% 89.38% -0.04% ========================================== Files 51 51 Lines 3650 3656 +6 ========================================== + Hits 3264 3268 +4 - Misses 386 388 +2 ```

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[MImmesberger]

This one is finished!

[MImmesberger]

Thanks! Just added tests for the new lines and will merge after tests ran through.