RangeIndex Error caused by marginal_cost.py

[Tondi001]

I believe the marginal_cost.py file should be updated to add ", index=marginal_cost_df.index" to the
"df1 = pd.DataFrame(columns=coal_gens.name.values) for all df1 thru df4. In otherwords it should look like:

"df1 = pd.DataFrame(columns=coal_gens.name.values, index=marginal_cost_df.index)"

Otherwise, I get a range index error. H/T to Yifan on Dr. Wei Sun's team.