Remove Scenario.get_optimum

One could remove the method Scenario.get_optimum which only returns the value of the public attribute Scenario.optimization_result. By the way, an OptimizationResult is more than an optimum; it also contains metadata related to the optimization.

    def get_optimum(self) -> OptimizationResult | None:
        """Return the optimization results.

        Returns:
            The optimal solution found by the scenario if executed,
            ``None`` otherwise.
        """
        return self.optimization_result