Transitive Reduction: standalone vertices are being removed from the graph

[nicolas-2008]

TransitiveReduction has an issue working with graph which contains not linked vertices.

            var graph = new BidirectionalGraph<string, Edge<string>>();
            graph.AddVertex("/test");
            graph.AddVertex("/test/123");
            graph.AddVertex("/test/notlinked");
            graph.AddEdge(new Edge<string>("/test", "/test/123"));

            var reduced = graph.ComputeTransitiveReduction();

Only two vertices are left in reduced graph.

[KeRNeLith]

I made a fix for this in my fork of the library (with also a lot of others fixes and .NET Core compliance).
It is available in latest release (after this commit).