Workbench module that fills in missing values.

First, get up and running:

1. pip3 install pipenv
2. pipenv sync # to download dependencies
3. pipenv run python ./test_fillna.py # to test

To add a feature on the Python side:

1. Write a test in test_fillna.py
2. Run pipenv run python ./test_fillna.py to prove it breaks
3. Edit fillna.py to make the test pass
4. Run pipenv run python ./test_fillna.py to prove it works
5. Commit and submit a pull request

To develop continuously on Workbench:

1. Check this code out in a sibling directory to your checked-out Workbench code
2. Start Workbench with bin/dev start
3. In a separate tab in the Workbench directory, run pipenv run ./manage.py develop-module ../fillna https://github.com/CJWorkbench/fillna.git
4. Edit this code; the module will be reloaded in Workbench immediately
5. When viewing the module in Workbench, modify parameters to re-render output