Extractors are mappings that extract data from a PageNode.
Necessary to get some type safety in CompositeExtractor:
Class[_ <: Extractor] can be checked at runtime, but Class[_ <: Mapping[PageNode]] can not.
Datasets generated by this extractor.
Datasets generated by this extractor. Used for serialization. If a mapping implementation
does not return all datasets it produces, serialization may fail.
The subject URI of the generated triples
A graph holding the extracted data
when extractor needs some finalization
when extractor has a pre-phase