Feat/sampling extractors

[manon-but-yes]

Related issues: \

Make the structure of the sampling function more general to allow the user to specify the estimator for the expectations and reproducibility. Also propose some implementations a few commonly used estimators.

This PR introduces:

• Modify the sampling function to take two additional arguments: the fitness extractor and the descriptor extractor
• Provide 4 expectation extractors: average, median, mode and closest, and 3 reproducibility extractors: std, mad and iqr
• Define a new sampling_reproducibility function that returns both the expectations and reproducibility of fitness and descriptor
• Update the sampling testing to test these additions

Checks

• a clear description of the PR has been added
• sufficient tests have been written
• relevant section added to the documentation
• example notebook added to the repo
• clean docstrings and comments have been written
• if any issue/observation has been discovered, a new issue has been opened

Future improvements

The functions used by MAP-Elites Low-spread could be define as extractor in sampling.py as well to avoid code duplication.