[MAINT] Run sklearn check_estimators on LOCO and D0CRT

[jpaillard]

• Remove _check_fit to use the sklearn convention __sklearn_is_fitted__ instead. See https://scikit-learn.org/stable/auto_examples/developing_estimators/sklearn_is_fitted.html#sphx-glr-auto-examples-developing-estimators-sklearn-is-fitted-py
• Move the check_estimator tests for all classes in the test_sklearn_estimators.py module to avoid duplicated code.
• Add check_estimator test for LOCO and D0CRT

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 98.33%. Comparing base (8b51c10) to head (e9addd9).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #611      +/-   ##
==========================================
+ Coverage   98.16%   98.33%   +0.17%     
==========================================
  Files          23       23              
  Lines        1634     1626       -8     
==========================================
- Hits         1604     1599       -5     
+ Misses         30       27       -3     

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