[EXTREME ESTIMATOR][MARGIN MODEL] add ismev gev fit. and refactor. add result_from_fit abstract class.

[EXTREME ESTIMATOR][MARGIN MODEL] add stationary/non-stationary margin model. fix margin formula (in the temporal case when we want a dependency on a single parameter for instance). add margin function with starting point

[EXTREME FIT] After any Spatial Extreme fit, create an object ResultFromFit, refactor code accordingly

[EXTREME ESTIMATOR][EXTREME MODEL] add warning when maximum absolute value of data is too high in safe_run_r_estimator. add test.