Add additional TraversalOutputFormat for summary only

[nreinicke]

Currently, if we run a search with the traversal output plugin but do not set the route key like: output_plugins = [ type = "traversal"}], we will not get any route results. Sometimes we might want to run a query but not care about the actual geometry or any of the link level statistics. We could add a new TraversalOutputFormat with a name like Summary that only returns the traversal summary for each route and nothing else.