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

# :rocket:

- [ ] 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.