jupyter notebook visualizations and Odin error messages

Changes

• jupyter notebook visualizations for Sentence graphs (dependency parses) and Mention structure
  • visualization.JupyterVisualizer.display_graph()
  • visualization.Jupyter.Visualizer.display_mention()
• Display error messages for broken Odin rules
• Upgraded processors-server version to v3.0.2
• Added HeadFinder and HeadFinder.semantic_head()
• pagerank bug fix related to outdated parameter
• Make Sentence, DirectedGraph, and Mention hashable