Add support for nonlinear value coupling

[KoraTMontemagno]

I tried to update the functions using non linear equations I don't know if I handled correctly the static arguments though...

[codecov-commenter]

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Codecov Report

Attention: Patch coverage is 94.28571% with 2 lines in your changes missing coverage. Please review.

Project coverage is 95.05%. Comparing base (f943078) to head (85e35e8).

Files	Patch %	Lines
src/pyhgf/model/network.py	90.90%	1 Missing :warning:
src/pyhgf/utils.py	66.66%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #215      +/-   ##
==========================================
- Coverage   95.10%   95.05%   -0.05%     
==========================================
  Files          23       23              
  Lines        1184     1213      +29     
==========================================
+ Hits         1126     1153      +27     
- Misses         58       60       +2     

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[LegrandNico]

Alright, I think we can merge here, thank you for your work @KoraTMontemagno