[DOC, ENH] Cleanup repo and docs and add typing

[adam2392]

First part of #7

Changes proposed in this pull request:

Pretty much this just moves files around and modifies the build to rely on the networkx PR for MixedEdgeGraph.

• updates poetry build for dependencies
• moves graph files into classes/
• adds the Node type, which is synonymous with dodiscover's Column type. Just makes typing with mypy easier on the repo.

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