def _rdf_graph_from_ntriples_string(ntriples_string, graph=nil)
      graph ||= RDF::Graph.new
      RDF::Reader.for(:ntriples).new(ntriples_string) do |reader|
        reader.each_statement do |statement|
          graph << statement
        end
      end
      graph
    end
def _rdf_graph_from_ntriples_string(ntriples_string, graph=nil)
      graph ||= RDF::Graph.new
      RDF::Reader.for(:ntriples).new(ntriples_string) do |reader|
        reader.each_statement do |statement|
          graph << statement
        end
      end
      graph
    end
given a string of ntriples data, populate an RDF graph.
 If you pass a graph in, it will add to that one.