Illustrate how to feed custom topologies

[redeboer]

In principle, it should be possible to feed custom decay topologies to the system, which would for instance be useful for re-scattering processes. The interfaces may need some more entry points (not sure if the StateTransitionManager is flexible enough already). At any rate, the solving module should already be able to handle a custom set of topologies -- it will just propagate quantum numbers over the edges and nodes.

So two things need to be done:

1. Add a notebook that illustrates how to handle some (let's say) re-scattering process
2. (Probably:) further expose some of the procedures from the StateTransisionManager.

(It could even be that 2. has to be addressed through #20, that is, splitting up the STM into smaller classes or functions, to expose each of its steps.)