ReducedFunctional should provide dual space derivatives

[jrmaddison]

Optimizers need access to dual space derivatives, and then the correct inner product should be used elsewhere, e.g. handing Riesz mapped derivatives to an optimizer doesn't work, #153. Currently ReducedFunctional.derivative and ReducedFunctional.hessian return Riesz mapped derivatives. This is worked around in #143 by discarding the return values and accessing adjoint variables directly.