Improve symmetry reduction

[blegat]

• Fix block diagonalization Fix semisimple -> simple in symmetry reduction #308
• Add dihedral example back Fix semisimple -> simple in symmetry reduction #308
• Add block diagonalization to SymbolicWeddernurn.jl as fallback
• Add symmetry reduction equality to PolyJuMP
• Use semisimple = false
• Probably use droptol like in PropertyT