Add function to compute CO2-eq emissions

[miloknowles]

See calculate_co2_eq_mass in data_cleaning.py.

[miloknowles]

Also, added some tests for this function in test_data_cleaning.py. I installed pytest to run those tests, and accidentally started tracking my Pipfile* stuff as a result. Let me delete those since you're using conda.

[miloknowles]

Wait now I read the README and it looks like we do support pipenv, we just haven't created any environment files. I'm going to include the Pipfile and we should update it to include all of the dependencies in the future.