Add tests to ensure that data download and EIA-923 cleaning works for 2005-2020

[miloknowles]

This PR resolves some issues that were breaking the clean_eia923 function on older years.

• Fixes a missing primary fuels exception for plants with zero net generation and fuel consumption
• Handles inconsistent file naming for EIA-923 and EIA-860 data in historical years
• Adds a test to confirm that clean_eia923 runs without failing for 2005-2020.

Other improvements

• Since most of the functions in download_data.py duplicate the same code for downloading and unzipping data, I refactored them to use a download_helper function. All of the download functions have tests now, and I've confirmed that they still work after refactoring.
• Started on a "work in progress" notebook for parsing the EIA Electric Power Annual uncontrolled emission factors in case we want to use those

[miloknowles]

Also, I changed EIA-923 to allow users to disable subplant ID merging as well as NOx and SO2 calculations. The default values of those flags will cause the function to behave the same as before everywhere else.

[miloknowles]

For refererence, this PR advances #117