Z4c: fix bug related to the misuse of sym_symm

[lwJi]

Be careful when using sym_symm, make sure both of the two tensors are symmetric.
we can't use it for $\partial_c\gamma^{a(e}\gamma^{f)b}A_{ab}$, because $\partial_c\gamma^{a(e}\gamma^{f)b}$ is not symmetric in $(ab)$

[eschnett]

Did you check that the expressions themselves are correct, i.e. whether the implemented equations correspond to the ones in the paper? I might e.g. have changed the order of indices.

[lwJi]

Did you check that the expressions themselves are correct, i.e. whether the implemented equations correspond to the ones in the paper? I might e.g. have changed the order of indices.

I checked all the terms and compared them with the paper. I believe they now all agree. @eschnett