08_reach.ipynb: Add code to perform reachability queries

Currently supported reachability labels are only:
- backward topological levels (negative-cut filter)
- min-post tree intervals (positive-cut filter)
- min-post graph intervals (negative-cut filter),
  which is not yet produced by anything in `labelling.dfs_intervals`

There are three versions of reachability querying with the help of
reachability labels (all suport the same set of labels):
- recursive `generic_is_reachable()`, like Algorithm 3 from FELINE paper
- stack-using `generic_is_reachable_dfs()`
- queue-using `generic_is_reachable_bfs()`, split into few subroutines

All of those pass the basic tests.

Compared to the original Google Colab notebook the FELINE index support
and the support for extensions to min-post intervals from PReaCH got
removed for the time being.

Code cleaned-up by exporting it via `nbdev_build_lib` equivalent, then
using PyCharm 2020.3, finally `nbdev_update_lib` was used to update
the `08_reach.ipynb` notebook.

Author: jnareb

08_reach.ipynb

@@ -10,7 +10,13 @@
          "commit_graph": "03_example_graphs.ipynb",
          "find_levels": "06_levels.ipynb",
          "find_dfs_spanning": "07_interval_labels.ipynb",
-         "find_dfs_intervals": "07_interval_labels.ipynb"}
+         "find_dfs_intervals": "07_interval_labels.ipynb",
+         "generic_is_reachable": "08_reach.ipynb",
+         "generic_is_reachable_dfs": "08_reach.ipynb",
+         "reachable_positive_cut": "08_reach.ipynb",
+         "reachable_negative_cut": "08_reach.ipynb",
+         "walk_spanning": "08_reach.ipynb",
+         "generic_is_reachable_bfs": "08_reach.ipynb"}
 
 modules = ["example_graphs.py",
            "labelling/levels.py",