Remove generic template from StateTransitionGraph

[redeboer]

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.

[redeboer]

So a few problems:

• Currently, the combinatorics module correctly uses an edge property that is different from ParticleWithSpin, because the StateTransitionGraph is initially filled with particle names + spin projections (not Particle instances) only:
  https://github.com/ComPWA/expertsystem/blob/d45a3670e631ce85b66b652febf5903d1ccd5278/expertsystem/state/combinatorics.py#L144-L146
• There is unfortunately no way to set a default type on a typing.Generic or typing.TypeVar. It would be nice though if the default type were ParticleWithSpin so that you don't have to write StateTransitionGraph[ParticleWithSpin] in the rest of the framework.
• At most it's possible to set constraints on the TypeVar definition, but this does not really address the issue, just adds a layer of safety. For instance, there is still a [dict] here by mistake:
  https://github.com/ComPWA/expertsystem/blob/778541d8b64a4a66259b5421e578cd28f562b744/tests/unit/ui/test_system_control.py#L109

So I'll put this one back into the icebox. It would be better to get rid of this template altogether, but for that the interface will have to be changed (for instance in #20, see also description of ComPWA/expertsystem#314).

[redeboer]

See also #49

[redeboer]

Closed by #156