Statistics in MOABB

MOABB has a very good statistical methodology for experimental analysis and benchmarking algorithms.
It could be a nice addition to reinforce these aspects with:

• a specific example/tutorial, that details the chosen statistics and how to use it.
• improve definition of chance level

Regarding chance level, we simply use the limit when the number of samples tends to infinity (50% for 2 classes, 25% for 4 classes, etc). This chance level is a crude approximation and could be refined, either using the class sample balance (Better than random: a closer look on BCI results) or by characterizing the class sample distribution (Exceeding chance level by chance)