networkx: fix deprecation warning on 'node_link_graph'

close #62 (closed)

note: as neither 'link' or 'edges' was provided (parameter used to provide the ‘edges’ attribute name for storing NetworkX-internal graph data) simply adding the 'edges'="links" should be enough. (looking at the code we are not 'affected' by this deprecation warning, they kind of renamed 'link' to 'edges')