Reduce responsibilities of the `StateTransitionManager`

[redeboer]

The StateTransitionManager is supposed to serve as a facade to the expertsystem, but many of its methods (such as _determine_node_settings) perform quite a lot of tasks. That raises the question whether they shouldn't become functions elsewhere, so that the StateTransitionManager can be made smaller.

This issue has become related to ComPWA/expertsystem#314 / ComPWA/expertsystem#196.

• The reaction.combinatorics function can take more of the responsibilities that the StateTransitionManager performs. It is currently rather empty and it's unclear why it's a separate module.
• The StateTransitionManager can be stripped of its amplitude creation functionality and put directly under reaction. Some higher-level facades can then provide the functions that are needed for the quickstart.