Improving compile times

[sethaxen]

With #178 we added support for ADTypes, but using Optimization.jl to construct the gradient and hessian functions causes compile times for pathfinder to be very slow. This propagates also to extensions such as the Turing support.

We should try to figure out exactly why the compile times a now higher and reduce them. We could even define our own grad/hess functions if that helps.