Replace pulp.LpVariable() calls with prob.add_variable()

[martinlackner]

Summary

This PR refactors all variable creation in the PuLP-based voting rules to use the prob.add_variable() method instead of directly calling pulp.LpVariable(). This change improves code consistency and aligns with PuLP's recommended API for variable management within a problem context.

Key Changes

• Replaced all pulp.LpVariable() calls with prob.add_variable() throughout the module
• Updated variable creation in the following functions:
  • _enumerate_committees_standard_pulp() - committee membership variables
  • _enumerate_committees_lex_pulp() - committee and lexicographic optimization variables
  • set_opt_model_func() - utility, satisfaction, partition, load, and loadbound variables
  • _pulp_minimaxav() - Hamming distance and voter distance variables
  • set_opt_model() - voter utility variables
• Reformatted dictionary comprehension in _enumerate_committees_standard_pulp() for better readability

Implementation Details

• All variable creation now goes through the problem object's add_variable() method, ensuring proper variable registration with the problem instance
• This change maintains backward compatibility while improving code maintainability
• No functional changes to the optimization logic or constraints

https://claude.ai/code/session_01EKgeNsu9avVGurvMVkyCpV

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 72.72727% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.33%. Comparing base (da582c2) to head (6931c6e).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
abcvoting/abcrules_pulp.py	72.72%	3 Missing ⚠️
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Additional details and impacted files

@@           Coverage Diff           @@
##           master     #113   +/-   ##
=======================================
  Coverage   89.33%   89.33%           
=======================================
  Files          12       12           
  Lines        3563     3563           
  Branches      859      859           
=======================================
  Hits         3183     3183           
  Misses        257      257           
  Partials      123      123           

Files with missing lines	Coverage Δ
abcvoting/abcrules_pulp.py	78.19% <72.72%> (ø)

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