Add mongo_to_pg Python script and notebook

Author: kinow

docs/mongo-to-postgres/.gitignore

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+*.json
+*.csv
+.ipynb_checkpoints/
+venv/
+

docs/mongo-to-postgres/README.md

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+# CWL Viewer MongoDB migration to PostgreSQL
+
+This directory holds documentation that may be useful for other CWL Viewer
+developers (probably not for users.)
+
+## Spring Boot & Hibernate upgrade
+
+Issue: <https://github.com/common-workflow-language/cwlviewer/issues/254>
+
+## Jupyter Notebook, `mongo_to_pg.ipynb`
+
+Issue: <https://github.com/common-workflow-language/cwlviewer/issues/395>
+
+After upgrading the Spring Boot and Hibernate code, we were left with the
+production MongoDB database in AWS. Even though we could create a new
+environment with PostgreSQL, we still needed to migrate the production
+database.
+
+The first step to work on the issue was to obtain a sample of the data
+exported from the system. Instead of relying on someone being able to
+access MongoDB in AWS, the first try was via the existing `dump.sh` script
+(`load.py` is much slower.)
+
+With the data on disk, a Jupyter Notebook was created to process the data
+with Pandas, and produce a CSV to be `COPY`ed by PostgreSQL, importing
+the data directly into the `workflow` table (the only other table in
+CWL Viewer is `queued_workflow`, but it only holds data for workflows being
+processed, can probably be ignored.)
+
+The code of the Notebook is in this directory, and can be used to review
+what was done, or to modify it for other use cases. The Python script with
+the same name (but `.py` extension) was created based on the Notebook, to
+be used in the command line.
+
+To install the dependencies, use `pip install -r requirements.txt
+in a virtual environment to get the dependencies to run both the
+Notebook and the Python script.