Now that the nested dicts have been moved out (#297), it would be nice to remove the edge type template of the StateTransitionGraph:
https://github.com/ComPWA/expertsystem/blob/778541d8b64a4a66259b5421e578cd28f562b744/expertsystem/topology.py#L301-L304

Internally, the expertsystem (almost) only uses ParticleWithSpin as edge type. In addition, the fact that the node properties are not templated is counter-intuitive.