This closes #225

This adds an "add_missing" option to allow the automatic addition of
vertices if they do not exist in the graph.

This also changes the parameter "new" in Graph.edge() to "add_missing".

This closes #226

Improve slightly the performance of shortest_distance() with multiple targets and revert shortest_path() to single-target-only

update: topology.shortest_distance and topology.shortest_path to sear…

This closes #222 and #223.

This fixes #221

This fixes issue #220