Update Grid and Feedstock so prices can be provided per-year

Update Grid and Feedstock so prices can be provided per-year

@vijay092 requested this for the grid model, but I thought it was relevant for the feedstock model as well. Currently, both of these models can take price inputs in as either a single value or as a list/array that has a length equal to n_timesteps. It'd be nice if these options were expanded to include a case where the prices would be specified as a list/array with a length of plant_life.

This can be done pretty easily by updating the logic in GridCostModel like below (but it should also be updated for the electricity_buy_price shape)

if self.config.electricity_buy_price is not None:
    buy_price = self.config.electricity_buy_price
    if isinstance(buy_price, list | np.ndarray):
        if len(buy_price) != n_timesteps or len(buy_price) != plant_life:
            raise ValueError(
                f"electricity_buy_price length ({len(buy_price)}) "
                f"must match n_timesteps ({n_timesteps}) or plant_life ({plant_life})"
            )
        
        buy_price_shape = len(buy_price)
    else:
        buy_price_shape = 1

However, there would be some additional considerations to handle variable dt or varying simulation length cases. I will update this issue with that proposed solution later.