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.

Changed the export functionality of ZipEntryManager to exporting the
validation reports of validation entries, instead of exporting carbon
copies of the input Zip files' CityGML files.