Refactor Scenario building so that costs curves are created via heat-rate-curves and fuel prices

[danielolsen]

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• Is your feature request essential for your project?

Describe the workflow you want to enable

Currently, within the plant.csv data for a grid, we have heat-rate-curve coefficients, fuel prices, and cost curve coefficients, but only the cost curve coefficients are used. The origin of the existing cost curve coefficients is a bit murky, especially compared to the heat rate curve values that are being estimate from real EPA data during HIFLD grid-building. I wish instead of using the cost-curve-coefficients, we instead generated them on the fly during Scenario preparation by multiplying fuel prices against heat rate curves, and optionally explicitly adding non-fuel cost curve coefficients. This could also enable:

• During Scenario creation, the user can define a new fuel price assumption for some or all generators of a certain type.
• For CEM Scenarios, we have different fuel prices for different years that modify the cost curves for running generators in those years.

Describe your proposed implementation, if applicable

Basic:

• When instantiating a Grid object from CSVs, information from gencost.csv is ignored and the information is gathered instead from plant.csv.

Bonus:

• The ChangeTable would get a new function to modify the fuel price assumption for some or all generators of a certain type.
• TransformGrid interprets the new change table keys and generates new gencost values.