Be selective about re-investments in the ironing out loop

We currently redo all investments each round of the ironing out loop. Once #787 is done, this means that we will redo __all__ investments if the price of __any__ commodity is unstable. This is similar to what we do in MUSE1.

However, we could be more selective about this and only redo investments that could possibly be impacted by the unstable prices.

E.g I think the means that if the price of electricity changes from one iteration to the next, we should redo investments for all commodities that could consume electricity in the process of their production. Then, if any of those investments change compared to the previous iteration, we would have to redo the investments upstream of this (including electricity itself)

Let's say we had a very simple model:

`gas -> electricity -> heat`

- In round 1 we perform investments for `heat` then `electricity` then `gas` based on commodity prices from the previous milestone year.
- After doing all investments and then a final dispatch run to get new prices, we notice that the price of `electricity` has changed compared to the previous milestone year, so we have to redo investments for `heat` which might be impacted by this price change*
- If `heat` investments are _unchanged_ compared to round 1 then we're done (assuming the `gas` price is also stable)
- If `heat` investments are _changed_ compared to round 1 (perhaps towards a more electricity-efficient technology), then we'll have to redo investments for `electricity`, and likewise if these change then we'll need to redo investments for `gas`
- After this new round of investments, we do another full dispatch run to get a new set of commodity prices, etc. etc.
- This keeps going until the price of all commodities is stable, or the max number of iterations is reached

*Or, perhaps if the price of `electricity` has changed then this will be factored in to the price of `heat` so we don't need to look forward and can just redo investments for the commodities with changing prices, starting with those furthest along the supply chain?

@ahawkes Does this sound reasonable?