Function add_marginal_cost_average_prices 

fn add_marginal_cost_average_prices<'a, I, J>(
    activity_for_existing: I,
    activity_keys_for_candidates: J,
    existing_prices: &mut CommodityPrices,
    year: u32,
    markets_to_price: &HashSet<(CommodityID, RegionID)>,
    commodities: &CommodityMap,
    time_slice_info: &TimeSliceInfo,
)
where
    I: Iterator<Item = (&'a AssetRef, &'a TimeSliceID, Activity)>,
    J: Iterator<Item = (&'a AssetRef, &'a TimeSliceID)>,

Calculate marginal cost prices for a set of commodities using a load-weighted average across assets and add to an existing prices map.

Similar to add_marginal_cost_prices, but takes a weighted average across assets according to output rather than taking the max.

Candidate assets are treated the same way as in add_marginal_cost_prices (i.e. take the min across candidate assets).