[DATASET] add creation of spatio temporal dataset from simulation. add test for non stationarity of max stable

[TEST][DATASET] major fix for abstract_dataset for the function transform_maxima in the spatio temporal case

[SIMULATION] add constant estimators. handle graph plot of constant margin estimator (when the sampling margin was also constant)

[MAX STABLE MODEL] add fitmaxstab for 1D coordinates (by building 2D coordinates from the 1D coordinates)