Add Polynomial Optimization back

[blegat]

With https://github.com/jump-dev/SumOfSquares.jl/pull/282, increasing maxdegree don't change anything so the examples does not work anymore. It was not totally correct anyway since the minimizer can be recovered at degree 3 already using (6.15) of Laurent, Monique. "Sums of squares, moment matrices and optimization over polynomials." Emerging applications of algebraic geometry (2009): 157-270.

[blegat]

At degree 3, we find (0.5, 0.5) actually so we need degree 4