Dependency graph traversal

[Samuel-amap]

Currently, dependency graph traversal is preorder, meaning the current node is visited before its children. This means a node with two parents will be visited twice, and so forth.

This isn't an issue for the run! functions as the simulation_id prevents nodes from being run! multiple times per timestep, but it is behaviour worth documenting / extending, as it goes against typical expectations.

[VEZY]

This is closed in release v0.13.2 by adding the nodes_visited argument to the traversal function, which is a set of all nodes already traversed. We then test if the current node is inside this set, and if not we apply the function.