REFACTOR: restucture the MapieQuantileRegressor Fit - Split the fit into prefit_estimators, fit_estimators and conformalize

[jawadhussein462]

Description

This PR refactors the MapieQuantileRegressor class by restructuring its fit method into three distinct steps:

1. prefit_estimators: Used when the quantile estimators are already pre-fitted. This ensures that existing fitted estimators are reused without additional training.
2. fit_estimators: Used when the quantile estimators are not pre-fitted. This step trains the estimators from scratch.
3. conformalize: Applies conformal prediction adjustments and computes residuals errors.

Type of change

Please remove options that are irrelevant.

• New feature (non-breaking change which adds functionality)

Checklist

• I have read the contributing guidelines
• I have updated the HISTORY.rst and AUTHORS.rst files
• Linting passes successfully : make lint
• Typing passes successfully : make type-check
• Unit tests pass successfully : make tests
• Coverage is 100% : make coverage
• Documentation builds successfully and without warnings : make doc

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (614293e) to head (dc4fc26).
Report is 825 commits behind head on master.

Additional details and impacted files

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  Coverage   100.00%   100.00%             
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  Files           39        62     +23     
  Lines         4616      6006   +1390     
  Branches       487       450     -37     
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+ Hits          4616      6006   +1390     

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