Fixed ordering of `fixup_halos` and `halo_exhange.execute_adjoint` in `StructuredColumns.cc`

[odlomax]

Whilst integrating util::pack_vector_fields into the SphericalVector interpolation method, I uncovered a bug in StructuredColumns. The methods fixup_halos and halo_exchange.execute_adjoint were called in the wrong order during a halo exchange adjoint. This PR fixes the bug.

I'd initially missed this by adding (and then forgetting) a workaround in the SphericalVector test.

@MarekWlasak this will likely be of interest to you.