Checkable now checks if an accepted Visitor is flagged as a Validator
before setting the object as validated=true. Allows the implementation
of Visitors for the Check-Engine that serve as utilities, e.g. to enable
decoupled collection of all contained Geometries in a CityObject.

Version 3.17.0 Release

See merge request !28

TransportationObject model rework

See merge request !27

Changes the construction of the TreeLists and the renderer handling to
reflect the new model of TransportationObjects.