refactor: rename parameters to parameter_defaults

[redeboer]

This naming makes more sense, as HelicityModel.parameters sounds like some sequence that lists the parameters in the model, while it's actually a mapping of those parameters to default ('suggested') values.

[codecov]

Codecov Report

Merging #531 (2d9cdfb) into master (d5eceff) will increase coverage by 0.01%.
The diff coverage is 76.92%.

@@            Coverage Diff             @@
##           master     #531      +/-   ##
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+ Coverage   86.10%   86.11%   +0.01%     
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  Files          22       22              
  Lines        3584     3587       +3     
  Branches      876      876              
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+ Hits         3086     3089       +3     
  Misses        319      319              
  Partials      179      179              

Flag	Coverage Δ
unittests	86.11% <76.92%> (+0.01%)	⬆️

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Impacted Files	Coverage Δ
src/expertsystem/amplitude/helicity.py	85.66% <76.92%> (+0.13%)	⬆️

[spflueger]

Sounds reasonable here. Mostly because these parameters are not connected to the model. Only if you do some action like subs. I want to point out though, that in the tensorwaves Model interface, the parameters are actually connected to the model. So there the name defaults would be wrong.

Also I should add, the crucial information of that dictionary is the names of the parameters. So that the distinction between parameters and variables of the model can be made. The default values are more some convenience feature