classes "Edge" and "Node" added. function "IsConnected(Graph) added and tested"

The classes "Edge" and "Node" will be used to refactor the class Graph.
So far they were added, but nowwhere used.
A function implementing the algorithm checking, whether a graph is
connected, was added. It shell be incorporated into the Graph class, but
i do not know yet, whether it should be static or member method.