GraphCollection

Completely re-written to be simpler and hopefully increase performance. Currently only supports bi-directional nodes.

##Example usage
How to graph the example on wikipedia

    var one = new GraphNode<int>(1);
    var two = new GraphNode<int>(2);
    var three = new GraphNode<int>(3);
    var four = new GraphNode<int>(4);
    var five = new GraphNode<int>(5);
    var six = new GraphNode<int>(6);
    
    one.AddNeighbour(six, 14);
    one.AddNeighbour(three, 9);
    one.AddNeighbour(two, 7);
    
    two.AddNeighbour(three, 10);
    two.AddNeighbour(four, 15);
    
    three.AddNeighbour(six, 2);
    three.AddNeighbour(four, 11);
    
    four.AddNeighbour(five, 6);
    
    five.AddNeighbour(six, 9);
    
    var graph = new List<GraphNode<int>> {one, two, three, four, five, six};
    
    var dijkstra = new Dijkstra<int>(graph);
    var path = dijkstra.FindShortestPathBetween(one, five);

Special thanks to Ikegami Atsuko http://cleo.ci.seikei.ac.jp/~atsuko/

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