Add Python version of script

[grgmiller]

I wanted to suggest that a Python version of the R script be developed (or that this code base be transitioned to Python) to help facilitate contributions by the parts of the user community that use the crosswalk but primarily use Python. Switching to Python might enable more robust contribution from the user community, most of whom I am aware of seem to exclusively publish code in Python. Examples:

• https://github.com/jdechalendar/gridemissions
• https://github.com/PowerGenome/PowerGenome
• https://github.com/catalyst-cooperative/pudl
• https://github.com/gschivley/carbon-index
• https://github.com/EmissionsIndex/Emissions-Index

At the very least, it might be helpful to specify an anaconda environment and python script/notebook that could be used to run the R script using a python package such as r2py.

[grgmiller]

Another example of a python-based use of the crosswalk: https://github.com/singularity-energy/open-grid-emissions