Renaming and restructuring of piecewise_polynomial parameters

[LauraGergeleit]

What problem do you want to solve?

Closes ##444. We should think about a renaming of the parameters used for piecewise_polynomial, so that we use piecewise_polynomial internally but 'hide' it from the parameter files.
Then, we could also avoid to specify values in the parameter file that are not specifically documented in the law but used in piecewise_polynomial, as it is the case for soli_st (s. ##444).

Remaining problem with 1: upper_threshold in soli_st:
While the threshold is overwritten with the correct value, the intercept_at_lower_threshold is calculated using the value specified in the parameter file. Thus, the test for soli_st still fail, when changing the parameter in the yaml-file.

[codecov]

Codecov Report

Base: 91.41% // Head: 92.96% // Increases project coverage by +1.55% 🎉

Coverage data is based on head (3fae675) compared to base (53d665c).
Patch coverage: 100.00% of modified lines in pull request are covered.

❗ Current head 3fae675 differs from pull request most recent head c3d4e74. Consider uploading reports for the commit c3d4e74 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #462      +/-   ##
==========================================
+ Coverage   91.41%   92.96%   +1.55%     
==========================================
  Files          76       74       -2     
  Lines        4005     3937      -68     
==========================================
- Hits         3661     3660       -1     
+ Misses        344      277      -67     

Impacted Files	Coverage Δ
gettsim/policy_environment.py	100.00% <100.00%> (ø)
dashboard/pre_processing_data.py	0.00% <0.00%> (ø)
src/_gettsim/visualization.py
src/_gettsim/transfers/unterhalt.py
src/_gettsim_tests/test_docs.py
...tsim/social_insurance_contributions/ges_pflegev.py
src/_gettsim/taxes/abgelt_st.py
...amic_pension_data/_create_custom_pension_params.py
src/_gettsim/demographic_vars.py
src/_gettsim_tests/__init__.py
... and 140 more

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[hmgaudecker]

Thanks ! Apologies, I was not clear enough I think.

What I had in mind, and that is hopefully in line with @mjbloemer's line of thinking, was to

1. only specify the parameters in the law in soli_st.yaml. From the top of my head, I might call them rate, tatsächliche_max_rate, freigrenze (not married to any of those)
2. We'll call an internal function that translates these parameters to the parameters for piecewise polynomial we had before. Machinery similar to functions that vary by year.

This way, you only need to look at the law to implement parameters for a new year, everything else is handled behind the scenes.

Note: The implementation does not need to be super-pretty now, I am just thinking hard about how to best unify the DAG to include functions like this, will be working hard to have that in during the first quarter of 2023, so don't worry too much about that part.

[hmgaudecker]

Oh, and in c83f860 I made those updates. Note that the structure in 1991 was much simpler, the current stuff only started appearing in 1995. Rest would have to be adjusted accordingly and we need a function translating that to the previously-defined parameters for piecewise_polynomial

[hmgaudecker]

Let's tackle this once we have implemented GEP 6 #471 or similar.