title: Convert to directed graph to undirected graph in Python igraph
body: <p>I am attempting to draw an undirected graph in iGraph, but the graph I draw seems to have little arrows on it still. Is it possible to convert the directed graph object to an undirected graph object, so that the arrows are gone?</p>
  
  <p><strong>Here's my code:</strong></p>
  
  <pre><code>import igraph
  import cairo
  import igraph
  graph = igraph.Graph.Read_Ncol('network.txt')
  igraph.plot(graph)
  </code></pre>
  
  <p><strong>network.txt =</strong></p>
  
  <pre><code>1 2
  2 1
  2 3
  2 7
  3 1
  4 2
  4 6
  5 4
  5 6
  7 4
  7 8
  8 9
  9 7
  10 7
  10 8
  10 9
  11 2
  1 2
  3 4
  5 8
  12 3
  1 1
  </code></pre>