Remove dependency on event data format

At the moment the fatras kernel expects input data as a nested set of event vertices. This is an unnecessary restriction and tightly binds it to a specific event data model. Instead, it should be sufficient if the fatras kernel takes a single input particle as input and outputs a list of generated hits and a list of generated particles due to interactions. The event-data specific handling would then happen in the framework algorithm that know the explicit event data model used.