title: UndirectedGraph defining using JgraphT/Jgraph
body: <p>I can't find the constructor for a undirected graph, which is an interface... I have been surfing the docs... but don't know how do I initiate an <code>UndirectedGraph Interface</code>.</p>
  
  <p>I have tried with SimpleGraph but it isn't working. Casting a simpleGraph to undirectedGraph isen't working...</p>
  
  <p>Any help is appreciated,
  thank you.</p>