Draft: Add Jupyter notebook operator
This MR is the result of the spike to test the execution of Jupyter notebooks in Airflow using Skein and Papermill. It implements a NotebookOperator (inheriting from SimpleSkeinOperator) that executes a given notebook. The operator's parameters are the notebook itself, a packaged conda env to use as an environment for Skein, and the parameters for the notebook. This MR also implements a test DAG that uses the NotebookOperator.
This code is not final, it is intended to be reused when we prioritize implementing this functionality.