composer.json: bump rdf-interface to ^2

composer.json: require rdf-interface ^2.0.0-RC7

DatasetGettersTrait: get*Value() return type adjusted

GenericQuadIterator::current() and GenericTermIterator::current(): re…

…turn null if the iterator is invalid

composer.json: dev deps removed

DefaultGraph: make compatible with both rdfInterface ^1 and ^2

GenericQuadIterator: missing use statement added

composer.json: bump rdf-interface to ^1.0.0

NtriplesUtil::serialize() deals also with quads

GenericTermIterator added