Adds first iteration of the Illinois model

[samgdotson]

This PR adds a Snakemake workflow to the repository. This change facilitates parallel development by breaking out each step into an individual python script and using a simple input-output mechanism.

To review this PR, please

• pull the change branch (snakemake) using either the command line or the GUI
• checkout the change branch (snakemake)
• update the mamba/conda environment using conda env update --file environment.yml --prune
• update your .env file according to the new data in the .env.template (i.e., personal data for NREL)
• run the snakefile using (snakemake --cores=1)

Please ignore changes to the jupyter notebooks, for now. We will be retiring these notebooks in the near term.

This PR closes the following issues:

• Closes #3
• Closes #8
• Closes #6
• Closes #10
• Closes #16
• Closes #18

Known future issues:

• The representative year is not harmonized for the various cost data (e.g., some data is from 2021, others for 2024).
• The current model has no renewable energy maxima, leading to an unphysical amount of solar/wind being built.
• Cost data for existing generators is sparse (because NREL does not include it in the ATB, thus additional data is required). Specifically existing coal plants and nuclear plants.
• Harmonizing the technology names between NREL and EIA is handled poorly. This could be updated by simplifying the config file and the scripts retrieve_costs.py and retrieve_generators.py.
• There should be a README file describing the different configuration options in the config.yml file.

New issues will be created for the repository based on this initial scan.