Improve symmetry reduction

[blegat]

• [ ] Fix block diagonalization https://github.com/jump-dev/SumOfSquares.jl/pull/308
• [ ] Add dihedral example back https://github.com/jump-dev/SumOfSquares.jl/pull/308
• [ ] Add block diagonalization to SymbolicWeddernurn.jl as fallback
• [ ] Add symmetry reduction equality to PolyJuMP
• [ ] Use semisimple = false
• [ ] Probably use droptol like in PropertyT