Changed WallError to be aligned with parent class.
Added slots to the node class in LinkWalker.
Started work on wilson.py.
Filled out wilson.py a bit more, but still very in progress.
Some more fleshing out. Added an in-progress open_paths function to
the cell class.
Deleted the entirely unnecessary case dict in the Cell class.
Replaced it with a directions dict, which has same basic functionality.