[GEV] add computation of the mean & variance. modify gev_estimator. add function visualize_summary_of_annual_values_and_stationary_gev_fit: to quickly visualize the repartition of the main empirical indicators and corresponding stationary gev indicators.

[EXTREME PARAMETERS] create an attribute undefined_parameters in ExtremeParameters object, to avoid crash when a shape is < 0 for point while we want to display

[EXTREME ESTIMATOR] add margin_fits folder. add gpd params & gpdmle_fit. refactor code to handle gev and gpd together