maint: general structure

[clatapie]

This PR updates the general structure of ast_tree.py and fixes a issues related to the CustomFunctions class.

Some adjustments were needed due to the fact arguments can be different between the custom functions and the autogenerated one. In this case, the arguments within the custom functions are the ones used.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 79.43262% with 29 lines in your changes missing coverage. Please review.

Project coverage is 84.37%. Comparing base (d6b48b2) to head (a5b3ffa).

Files with missing lines	Patch %	Lines
src/pyconverter/xml2py/formatter.py	10.00%	9 Missing ⚠️
src/pyconverter/xml2py/custom_functions.py	86.44%	8 Missing ⚠️
src/pyconverter/xml2py/writer.py	70.83%	7 Missing ⚠️
src/pyconverter/xml2py/cli.py	25.00%	3 Missing ⚠️
src/pyconverter/xml2py/ast_tree.py	95.45%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #356      +/-   ##
==========================================
- Coverage   84.55%   84.37%   -0.18%     
==========================================
  Files          13       13              
  Lines        2337     2432      +95     
==========================================
+ Hits         1976     2052      +76     
- Misses        361      380      +19     

Flag	Coverage Δ
coverage	84.37% <79.43%> (-0.18%)	⬇️

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