Rethink tranching for divisible assets

[tsmbland]

For divisible assets, the current approach is to progressively select asset capacity in chunks, add these chunks together, then break the capacity up according to the unit size. As well as being very convoluted, we're also doing appraisal on potentially unrealistic "assets" - since tranche size and unit size are independent, you might, for example, have a unit size is 1 and a tranche capacity of 1.5, so you'd be appraising e.g. 1.5 wind turbines, which is a bit wrong. An easy fix might be to modify the tranche capacity (determined by get_demand_limiting_capacity) so that it's consistent with the unit size (i.e. rounding up), but I there might be better approaches:

• We could do away with tranching all together for divisible assets and just appraise one unit at a time (with fixed capacity). This way, we're appraising a realistic entity (not 1.5 wind turbines), and we'd go directly to the unit rather than growing a "parent" asset only to divide up again. This might be slow as it would lead to more appraisal rounds, but it's worth noting that when it comes to retaining already commissioned assets we do this one unit at a time, so I don't really see why we wouldn't do this when commissioning new assets in the first place.
• Use integer variables in the appraisal optimisation to ensure that capacities are a multiple of unit size. Probably the hardest but most efficient. I could also see this being adapted to asset retention, rather than the current approach of retaining one asset at a time

I'd say start with option 1, but consider option 3 for the future