[exceeding] fix code to create a qqplot. (sort only once the empirical...

[exceeding] fix code to create a qqplot. (sort only once the empirical quantiles have been computed). Create ALTITUDE_TO_GREY_MASSIF in utils to ensure that the exceedance plots are always done with the same massifs everywhere (and these massifs are only the one that are not grey for the non-stationary gev & gumbel)