Add a rng to the model and use it instead of using seed!

[Tortar]

Using a rng instead of using seed! and store it in the model is generally recommended (this is the approach we use in Agents.jl) so I will update the model to this new form if there are no objections

[Devetak]

Hey. I think I have a bit of an abjection. The idea of the model is that the source code can be also modified. For many applications the Shock is not enough (like for example the INET model changing the sampling), so it's better if the code is as simple as possible to allow for this without too much effort.