Improve handing of shadow_prices.bin

[dkyleward]

Right now, if you split zones, you have to update this file to match the new number of zones and their IDs. Improve the model code to first join shadow prices to the zone layer. This will add nulls to the shadow price vector for new zones and allow the model to proceed without crashing.

For official model updates, you'd want to update the bin file permanently, but for TIAs and project work, you shouldn't have to.