[periodic] Check if current implementation for box is correct

For box, we consider a "primary" and "secondary" DOFs, i.e. the secondary DOF is periodically corresponding to the primary one.

We add the value of the secondary DOF residual to the one of the primary one

 res[m.first] += res[m.second];

and enforce

jac[m.first][it.index()] += (*it)

This makes sense, I guess.

For the secondary DOF, we adapt the matrix entries such that the is a -1 for the periodic DOF and a 1 for the own DOF. All other entries are zero. I guess the idea is to set the secondary DOF values equal to the primary ones.

I think we should also adapt the residual of the secondary DOF such that

res[m.second] = curSol[m.second] - curSol[m.first];