Add LeafNode class

[khaledsulayman]

Currently, leaf node objects are being passed around as a list of dictionaries, but there seems to be a very clear outline in code for what a schema would look like for a LeafNode class.

[bbrowning]

Yes, this is a great chance to add standard Python classes or even Pydantic models to stop passing around these dicts everywhere.

[bbrowning]

An excellent candidate to live on this new leaf node class is code like leaf_node_path = leaf_node[0]["taxonomy_path"].replace("->", "_") - we have more than one place that needs this path, and this would be great as a method on the leaf node instance objects.