add skip_sigma kwarg in R2NModel and LMModel

[MaxenceGollier]

@dpo @MohamedLaghdafHABIBOULLAH
comment
should replace #239

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 84.43%. Comparing base (e0f214d) to head (2bc835a).
⚠️ Report is 205 commits behind head on master.

Additional details and impacted files

@@             Coverage Diff             @@
##           master     #241       +/-   ##
===========================================
+ Coverage   61.53%   84.43%   +22.90%     
===========================================
  Files          11       13        +2     
  Lines        1292     1632      +340     
===========================================
+ Hits          795     1378      +583     
+ Misses        497      254      -243     

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[Copilot]

Pull Request Overview

This PR adds a skip_sigma keyword argument to the obj functions of both R2NModel and LMModel classes. This allows callers to optionally exclude the sigma regularization term from the objective function computation.

• Added skip_sigma keyword parameter to objective functions in both model types
• Modified solver algorithms to use skip_sigma = true when computing model functions
• Provides conditional logic to return objective values with or without regularization

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
src/R2NModel.jl	Added skip_sigma kwarg to obj function with conditional return
src/LMModel.jl	Added skip_sigma kwarg to obj function with conditional return
src/R2N.jl	Updated solver to call obj with skip_sigma = true
src/LM_alg.jl	Updated solver to call obj with skip_sigma = true

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