add test for #92
modify test following get-context changes
in d4adeeabb6e267231b327772e13d6c43235c1682
closeness centrality of isolates should be 0
follow directed links with BFS on mixed graphs - fix #92
relocate and hide Jung's quirky in/outedges defs at use site
make sure parallel edges can all be selected in paths
change signature of BFS.from for conceptual coherence
allow getting radius for mixed graphs in light of #92