- Adds `graphviz` to dependencies
- Uses the `graphviz` package for visualizing the AnalysisTree (needs
  the `dot` etc. commands installed, usually available from the
  `graphviz` on most Linuxes)

The `visualize` method uses the `networkx` to graph the nodes as a tree.
Each node in the tree is currently shown with its ID and start time

closes one half of #19