Add a new type of intersection called 'Roundabout', and implement the functionality to handle it in the `TrafficAnalysisVisitor` class.
The 'Roundabout' intersection should reduce traffic by 30%, therefore make sure that the traffic value is adjusted by 0.7.

Also, there is a clear problem in the `visit` method of the `TrafficAnalysisVisitor` class: the visitor doesn't recur on the children of the intersection. Fix this problem.
