Tutorial needs an update

fit_mle_gradient_free() deprecated.
dGEV needs to be replaced by GeneralScaling.
for the second part, DependentScalingModel{dGEV, GaussianCopula} does not exist. A correlation structure must be defined.
for now the hessian is not implemented for DependentScalingModel so issue 65 must be handled first.

JuliaExtremes / IDFCurves.jl