Add Differential Equations

[ChrisRackauckas]

https://github.com/JuliaDiffEq/DiffEqBenchmarks.jl has some examples that can be stolen. It would be nice to track the interaction between differential equations and MCMC tooling.

[itsdfish]

Thanks for this idea. Given my lack of experience with differential equations, I think I can probably set up a simple model, such as the Lokta-Volterra model. Stan has a version. So this would allow a comparison of all currently supported samplers.