Implement exact methods for LOO

Branch feature/loo https://github.com/appliedAI-Initiative/pyDVL/compare/develop...feature/loo implements LOO for linear smoothers, for which a closed form solution is known. Finish that and test.

The work is on the 'feature/loo' branch, implementing Leave-One-Out (LOO) for linear smoothers. Start by examining that branch's changes to understand the closed-form solution. Look for existing tests related to LOO or linear smoothers to understand the testing pattern. The goal is to complete the implementation and ensure it is properly tested.

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