Define AD rules

[ElOceanografo]

Need to define AD rules that can differentiate through calls to a MarginalLogDensity. These are defined in the TMB paper, just need to be implemented here...

[ElOceanografo]

To be more specific, in section 4.1 of http://www.jstatsoft.org/v70/i05/. It's pretty dense, but since we have https://github.com/ElOceanografo/SparseInverseSubset.jl it should be feasible.