Closing property handles for external resources

Property handles referring to external resources (e.g. xml or csv files) can be created explicitly calling Property::openExtern() or implicitly by defining extents referring to external resources. When updating external files via property handles, data is usually written to file when destroying the access node (which is usually not the case when closing the property handle). In order to force writing data to file, one should rather call Property::closeAll() instead of Property::close().