TDR requires None column in Fuels_data

[Gabe Mantegna]

Currently, the TDR algorithm throws an error if Fuels_data.csv does not have a None column (for resources that do not use fuel). This should not be required, as GenX itself does not require this column.

cc @cfe316

[Jacob Schwartz]

@lbonaldo Presumably after loading it from the csv file, TDR only interacts with Fuels_data as a dataframe. We could abstract this to separate the steps of

1. Load raw data from a file
2. Preprocess data, i.e. add a None column
3. (optionally) parse to separate the time series itself from the co2-per-mmbtu numbers.
4. Present the data to the client function.

[Luca Bonaldo]

Yes, I think we should have that kind of abstraction for all the inputs. Let me work on that.