feat/parallelisation

[hlbotterman]

Related issue: link

Changes include:

• use of joblib in Comparator class to allow parallelisation over splits or imputers. The use of the method compare does not change. By default, the use_parallel is True, but the user can also use it with the False flag.
• refacto of test_comparator.py to only use pytest and pytest-mock + new methods
• rearrangement for the docs (analysis before api)