Issue and solution to the computation of the divcurl function in the python implementation.

[HarleyKelly]

On line 90 in the file structured.py there is an error in the computation of the divergence in the divcurl function.

The code should be np.add(dudx,dwdz,dudx).

As well as this the negative curl is computed, if this is intentional it might be worth adding a comment to explicitly say this.