Extend the access_different_date functionality for jahresanfang

[paulinaschroeder]

What problem do you want to solve?

Extend access_different_date to include jahresanfang to load parameters from the beginning of the year, as mentioned in #597.

Todo

• [x] Extend access_different_date to jahresanfang
• [x] Add a test and test data
• [x] Document PR in CHANGES.md

[codecov[bot]]

Codecov Report

Patch coverage: 100.00% and project coverage change: +0.16 :tada:

Comparison is base (2342234) 94.07% compared to head (bfc5f57) 94.23%.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #600 +/- ## ========================================== + Coverage 94.07% 94.23% +0.16% ========================================== Files 48 49 +1 Lines 2903 2986 +83 ========================================== + Hits 2731 2814 +83 Misses 172 172 ``` | [Impacted Files](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/600?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/config.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/600?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-c3JjL19nZXR0c2ltL2NvbmZpZy5weQ==) | `100.00% <100.00%> (ø)` | | | [src/\_gettsim/functions\_loader.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/600?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-c3JjL19nZXR0c2ltL2Z1bmN0aW9uc19sb2FkZXIucHk=) | `87.00% <100.00%> (+0.31%)` | :arrow_up: | | [src/\_gettsim/policy\_environment.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/600?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-c3JjL19nZXR0c2ltL3BvbGljeV9lbnZpcm9ubWVudC5weQ==) | `100.00% <100.00%> (ø)` | | | [src/\_gettsim/time\_conversion.py](https://app.codecov.io/gh/iza-institute-of-labor-economics/gettsim/pull/600?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-c3JjL19nZXR0c2ltL3RpbWVfY29udmVyc2lvbi5weQ==) | `100.00% <100.00%> (ø)` | |

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