Cleanup bugs in dev branch

[grgmiller]

Purpose

Fixes two small issues in the dev branch before merging v0.3.1 into main:

• Updates the setup instructions in the readme to include how to install OGE as part of your pipenv (since OGE has a dependency on itself, but oge is not included in the pipfile, this was leading to an import error when doing a fresh pipenv install)
• Fixes a column name error that was introduced in https://github.com/singularity-energy/open-grid-emissions/pull/339 and not checked

Testing

Ran the pipeline for 2019

Where to look

Usage Example/Visuals

Review estimate

5 min

Future work

Checklist

• [x] Update the documentation to reflect changes made in this PR
• [x] Format all updated python files using black
• [x] Clear outputs from all notebooks modified
• [x] Add docstrings and type hints to any new functions created