State graph displays incorrect graphs

Displayed state graph can show states with several outgoing transitions and only one incoming transition, without being part of a cycle, which is incorrect: a state visited only once can only have one outgoing transition.

This can be due to several reasons:
- The visual representation in JavaFX can be incorrectly updated, displaying "ghost transitions"
- The underlying data structure storing the graph can be incorrectly updated when the execution trace changes
- The computation of the state equivalency classes of the execution trace can be incorrect