Concept definition

template<typename actor_t, typename propagator_state_t, typename stepper_t, typename navigator_t, typename... Args>
concept Acts::Actor =  (ActorHasAct<actor_t, propagator_state_t, stepper_t,
                             navigator_t, Args...> ||
                 ActorHasAbort<actor_t, propagator_state_t, stepper_t,
                               navigator_t, Args...>) &&
                !ActorHasOldVoidInterface<actor_t, propagator_state_t,
                                          stepper_t, navigator_t, Args...>