10_checkpoint.ipynb: Save and restore graph and node data

Add functions to compute graph structure, and reachability labels, and
save them as DataFrame, or load it from saved file if it exists.
Additionally one can now recover graph from dataframe, and store
reachability labels as graph attributes.

The following functions were added:
- compute_cached_df() - the workhorse of individual compute_* functions
- compute_cached_graph_df() and compute_cached_reachability_labels_df()
- dataframe_to_reachability_labels(), companion to already existing
  dataframe_to_graph()
- compute_cached_graph() and compute_cached_reachability_labels()

Updated TODO list in index.ipynb notebook.

Minor changes include renaming `load_graph_df_from_file()` to
`load_df_from_file()` and adding `save_df_to_file()`.

Author: jnareb