ReducedFunctional.derivative_cb_pre and ReducedFunctional.derivative_cb_post are overwritten if derivative_components is passed

[miguelcoolchips]

I was looking at this part of the code in ReducedFunctional

https://github.com/dolfin-adjoint/pyadjoint/blob/master/pyadjoint/reduced_functional.py#L91-L97

and I saw that if the user passes a derivative_cb_post and a derivative_components to the constructor, the former gets overwritten. Is this the intended behavior?

[dham]

I think @colinjcotter may have been slightly careless when he wrote that. I would have thought that any callback functions passed in to the constructor should be composed with the functions added on those lines.