FIX: Improve Variable management in Circuit

[Samuelopez-ansys]

Description

Variable class refactoring. It should improve the usage of Object oriented programming, mainly in Circuit designs.

Issue linked

Close #6710

Checklist

• I have tested my changes locally.
• I have added necessary documentation or updated existing documentation.
• I have followed the coding style guidelines of this project.
• I have added appropriate tests (unit, integration, system).
• I have reviewed my changes before submitting this pull request.
• I have linked the issue or issues that are solved by the PR if any.
• I have agreed with the Contributor License Agreement (CLA).

[codecov]

Codecov Report

❌ Patch coverage is 68.52590% with 79 lines in your changes missing coverage. Please review.
✅ Project coverage is 57.45%. Comparing base (bb3143b) to head (fbdf93f).
⚠️ Report is 1 commits behind head on main.

❌ Your patch check has failed because the patch coverage (68.52%) is below the target coverage (85.00%). You can increase the patch coverage or adjust the target coverage.

Additional details and impacted files

@@             Coverage Diff             @@
##             main    #6742       +/-   ##
===========================================
- Coverage   83.27%   57.45%   -25.82%     
===========================================
  Files         246      246               
  Lines       77487    77551       +64     
===========================================
- Hits        64526    44558    -19968     
- Misses      12961    32993    +20032     

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