feat: add fuel cost and heat-rate curve to grid object

[ghost]

Adds fuel price and heat rate curves to the gen columns which are imported from the aux file, and drops them when creating the .mat file. These will be used for analysis of CO2 emissions.

[ghost]

This has been rebased and tested, and works as intended.

[rouille]

Any reason to extract GenIOB, GenIOC and GenIOD from 'USAGenbus_aux.pkl' and insert this information in the plant data frame if later on they are not used in carbon anlysis? It seems that only GenFuelCost is required for the carbon statistics.

[ghost]

We don't currently use these curve parameters directly, but we could use them to directly calculate the carbon emissions (rather than calculating the heat rate curve implictly by diving the cost curve by the fuel cost) and they could be very valuable in the future as we seek to expand the capabilities of our Scenarios. For example: we can model the impact of a given fuel price or a given carbon price, which would require re-calculating the gencost curves based on the original heat rate curves.

[rouille]

4 new columns are added to the plant data frame. These columns are dropped when creating the MATLAB case file. It works as intended.