Adding function attributes to GenericObjects 'LaneCenterLine'

GenericObjects 'LaneCenterLine' should contain a citygml_function attribute (e.g. DRIVING, SIDEWALK, etc.). Using lod0Network geometries these lines could be written as 'Road' objects. (Lane centerlines would also correspond to linear representations of CityGML3.0 TrafficSpaces in granularity = 'lane') 