struct PhasePoint
{
    List<double> x;

    PhasePoint(List<double x) {
        this.x = x;
    }
    PhasePoint(params double[] x) {
        this.x = x.ToList();
    }

    static PhasePoint opeartor+(PhasePoint p1, PhasePoint p2) {
        if (p1.x.Lenght != p2.x.Lenght)
            throw new InvalidOperationException("Phase points must have same dimension!");

        var result_x = new List<double>();
        for (int i = 0; i < p1.x.Length; i++)
            result_x.Add(p1.x.[i] + p2.x.[i]);
        return new PhasePoint(result_x);
    }
    //аналогично реализуешь остальные операторы
}

//потом в коде метода используешь PhasePoint так же как обычный double

public List<SolvingPoint> Met(DelegateFunc.Func func,double h,double x0, double xk) ->
public List<SolvingPoint> Met(DelegateFunc.Func func,PhasePoint h,PhasePoint x0, PhasePoint xk)
etc.