Algorithm for calculating commodity prices

[alexdewar]

At the time of writing, we're calculating commodity prices as the dual values for the commodity balance constraints. This doesn't seem to work in every case though, so a different approach will be needed.

I'm creating a discussion for this as the approach will likely evolve over time, so we don't want the discussion threads to be tied to any one issue. We can make a new issue when we have an idea for a better approach or to fix bugs with the existing implementation.

If we want to be able to easily allow the user to compare the results for different price calculation algos, it might be worth allowing them to choose from different price calculation options in the model config. Ideally we don't want to have to maintain this in the long term though, so if we can whittle it down to one that'll make things easier.

[alexdewar]

I've opened an issue for a new approach to calculating commodity prices: #433

[alexdewar]

We now get prices for all commodities and the price calculation is configurable (albeit there's a problem with one of the options: #677), so I think this is done