[feature] Add 2 DAG Algorithms: ancestors and descendants

[hydrosquall]

I went through the docs to find some algorithms for processing DAGs, and noticed these two were missing / marked as TODO. They're nearly identical ports of the Python algorithm, the only difference is a different syntax for working with Sets.

https://networkx.github.io/documentation/stable/_modules/networkx/algorithms/dag.html#descendants

Let me know if you'd like to see anything else before these get brought into the mainline branch.

Checklist

• [x] Unit tests
• [x] Docstrings
• [x] Referenced parent Python code