How to supply `marginal_cost` to the profit maximization SLC strategy?

[johnjasa]

There are many considerations here.
We want to be able to dispatch the least expensive technology in a hybrid system.

Currently, technologies might have a price directly (e.g. grid) or they might use feedstocks with a clear price (e.g. NG plant with an NG feedstock).
More complicatedly, there may be additional costs associated with running technologies that are not captured in just those prices.
For instance, the VarOpEx comes into play, especially for components that model degradation or replacement costs.
Also, CapEx and OpEx may come into play (but perhaps they don't just for marginal cost), but we should discuss this.

We could have a config class for the SLC here with an entry like this in the plant_config (per @elenya-grant's suggestion):

  cost_per_tech:
    ng: 0.05  # $/kWh — NG dispatch cost; overridden in run script
    grid: buy_price
    wind: VarOpEx # averaged marginal cost at the SLC level using electricity_out

[kbrunik]

This cost_per_tech config class would then allow a user to set a marginal_cost that acts as a weight at the config level (e.g., ng in this case) which doesn't propagate to the finance models.

The user could also use buy_price for Grid which is also price for Feedstock components. This can be an array or a single value. There would then be a method in SLC to calculate the total cost of energy per timestep in profit max class in SLC. This would likely be a separate method, maybe something like feedstock_price_marginal_cost() or something snappier. That would do the dot product of the energy purchased and the price. This will ultimately propagate to the finance model as a feedstock and buy price $/{commodity_ammount_unit}/timestep.

The user could also use VarOpEx which is an output of the CostModelBaseClass and the {commodity}_out to calculate another type of marginal cost. The VarOpEx has a default value of zero in the CostModelBaseClass but can be computed within the individual cost models and is output in $/yr. We would need another internal calculation in the SLC profit max class, something like varopex_marginal_cost() to get it into $/{commodity_ammount_unit}/timestep.

[johnjasa]

I've implemented this in #732 based on guidance from your comment. Thank you!