feat(synthese): small refact of the code

[jacquesfize]

This PR proposes to reorganise the synthese python code.

[codecov]

Codecov Report

Attention: Patch coverage is 92.49617% with 49 lines in your changes missing coverage. Please review.

Project coverage is 84.07%. Comparing base (e4157cc) to head (139c566).
Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
...d/geonature/core/gn_synthese/blueprints/reports.py	87.24%	19 Missing ⚠️
...eonature/core/gn_synthese/blueprints/taxon_info.py	87.27%	14 Missing ⚠️
...d/geonature/core/gn_synthese/blueprints/exports.py	94.77%	8 Missing ⚠️
...nd/geonature/core/gn_synthese/blueprints/others.py	86.11%	5 Missing ⚠️
.../geonature/core/gn_synthese/blueprints/synthese.py	98.40%	2 Missing ⚠️
...eonature/core/gn_synthese/blueprints/statistics.py	98.33%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #3301      +/-   ##
===========================================
+ Coverage    83.91%   84.07%   +0.15%     
===========================================
  Files          122      129       +7     
  Lines         9698     9794      +96     
===========================================
+ Hits          8138     8234      +96     
  Misses        1560     1560              

Flag	Coverage Δ
pytest	84.07% <92.49%> (+0.15%)	⬆️

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