title: igraph layout_reingold_tilford gives errors
body: <p>I get the following error message when trying to use the  layout_reingold_tilford layout</p>
  
  <p>File "C:\Python27\lib\site-packages\igraph\layout.py", line 80, in <strong>init</strong>
      self._coords = [list(coord) for coord in coords]
  TypeError: 'int' object is not iterable</p>
  
  <p>I have found the following question which has a simple question and answer but when I try the example I get the same error</p>
  
  <p><a href="https://stackoverflow.com/questions/20213956/plot-a-tree-like-graph-with-root-node-at-the-top">Plot a tree-like graph with root node at the top</a></p>
  
  <pre><code>import igraph as ig
  g = ig.Graph(n = 12, directed=True)
  g.add_edges([(1,0),(2,1), (3,2), (4,3),
           (5,1),
           (6,2), (7,6), (8,7),
           (9,0),
           (10,0), (11,10)])
  g.vs["label"] = ["A", "B", "A", "B", "C", "F", "C", "B", "D", "C", "D", "F"]
  layout = g.layout_reingold_tilford(mode="in", root=0)
  ig.plot(g, layout=layout)
  </code></pre>