title: Graph_tool 's find_vertex() raises a Exception
body: <p>I generate a graph with the word of a sentence as tokens and dependency relations as edges.</p>
  
  <p>This info comes from a external Tool in the format of ((word, index), (word, index),  relation) tuple. Each word can have many dependencies and I want only one node per word.</p>
  
  <p>When I process each dependency I want to search it by their ID, but when i Try to use find_vertex() i get the next Exception:</p>
  
  <pre><code>TypeError: No registered converter was able to extract a C++ reference to type graph_tool::GraphInterface from this Python object of type Graph
  </code></pre>
  
  <p>This is the code:</p>
  
  <pre><code>def graph(self, sentence, sentenceNamespace="sentence", separator="_"):
      graph = Graph()
      root_node = graph.add_vertex()
      # Create a link to Root Node
      graph.graph_properties['root'] = graph.new_graph_property("python::object", root_node)
      # Create Node's properties
      graph.vertex_properties['id'] = graph.new_vertex_property("string")
      graph.vertex_properties['form'] = graph.new_vertex_property("string")
      graph.vertex_properties['ord'] = graph.new_vertex_property("int64_t")
      # Create edge's properties
      graph.edge_properties['type'] = graph.new_edge_property("string")
      # Create the root node
      graph.vertex_properties['form'][root_node] = "S"
      graph.vertex_properties['ord'][root_node] = -1
      # Find or create the dependant and governor nodes and link with a new edge
      for dependency in self.dependency(sentence):
          dependency_governor = dependency[0]
          # Governor's generated ID
          governor_name = "{1}{0}{2[0]}{0}{1[1]}".format(separator, sentenceNamespace,dependency_governor )
          dependency_dependant = dependency[1]
          # Dependant's generated ID
          dependant_name = "{1}{0}{2[0]}{0}{1[1]}".format(separator, sentenceNamespace, dependency_dependant)
          # Relation Type
          relation = dependency[2]
  
  # This is the problematic Line ################################################
  
          governor = find_vertex(graph, graph.vertex_properties['id'], governor_name)
  
  ##############################################################
          # If search is successful use the first node else create a new one
          if governor :
              governor = governor[0]
          else:
              governor = graph.add_vertex()
              graph.vertex_properties['id'][governor] = governor_name
              graph.vertex_properties['form'][governor] = dependency_governor[0]
              graph.vertex_properties['ord'][governor] = dependency_governor[1]
  
          dependant = find_vertex(graph, 'id', dependant_name)
          # If search is successful use the first node else create a new one
          if dependant :
              dependant = dependant[0]
          else:
              dependant = graph.add_vertex()
              graph.vertex_properties['id'][dependant] = dependant_name
              graph.vertex_properties['form'][dependant] = dependant_dependant[0]
              graph.vertex_properties['ord'][dependant] = dependency_dependant[1]
          # Link the nodes with the new dependency relation
          dependency_edge = graph.add_edge(governor, dependant)
          graph.edge_properties['type'][dependency_edge]= relation
      return graph
  </code></pre>