Some graph constructors create directed graphs by default, some create undirected ones, even when they can create either. This is confusing. Can we make the default undirected for all graph constructors in 2.0, when this makes sense? Opinions?

Examples

• These are all undirected by default, even though they can create directed graphs as well: sample_gnm, make_ring, make_chordal_ring, make_full_graph, make_bipartite_graph (!) ...
• These are all directed by default, even though they can create undirected graphs as well: make_tree, make_star, make_graph (!, compare make_bipartite_graph), ...
• Of course, there are cases when there is only one reasonable choice for the directedness. make_kautz must be directed, make_from_prufer must be undirected, make_line_graph must follow the directness of its argument, make_directed_graph and make_undirected_graph must follow their naming, etc.