Possible bug with hints generation

#90 encountered the following error during development.

```
AttributeError: 'Graph' object has no attribute 'pred'
Traceback:

File "python3.10/site-packages/grandcypher/__init__.py", line 1708, in run
return self._transformer._executors[0].returns()
File "python3.10/site-packages/grandcypher/__init__.py", line 897, in returns
results = self._lookup(
File "python3.10/site-packages/grandcypher/__init__.py", line 675, in _lookup
true_matches = self._get_true_matches()
File "python3.10/site-packages/grandcypher/__init__.py", line 1102, in _get_true_matches
for match in self._matches_iter(my_motif):
File "python3.10/site-packages/grandcypher/__init__.py", line 1190, in _matches_iter
for match in grandiso_finder:
File "python3.10/site-packages/grandiso/__init__.py", line 427, in find_motifs_iter
yield from walk(path)
File "python3.10/site-packages/grandiso/__init__.py", line 412, in walk
for candidate in get_next_backbone_candidates(
File "python3.10/site-packages/grandiso/__init__.py", line 260, in get_next_backbone_candidates
candidate_nodes_from_this_edge = host.pred[backbone[target]]
```

[This fix](https://github.com/aplbrain/grand-cypher/pull/90/files#diff-5d1eaef8dbb5966e005f064b9b23981dc3f48f9dd97f8cda59b06b3353912a3bR1150) worked around the problem but I'm unsure if that is the best approach. It appears somewhere along the way the new logic is incorrectly assuming the Graph is a DiGraph.