Investment issues (umbrella)

Main issues:

- #1358 (see also #1199 and #1357)We don't have an agreed upon approach for capacity/tranche size selection. Unclear whether capacities should be defined upfront or dynamic (there are software design implications here, and I'd advise against mixing approaches to keep things relatively simple). Unclear how this should interact with divisible assets. Unclear what the user interface should be.
- #1383 (see also #1319) We're appraising capacities in each tranche as if they're entirely separate assets, rather than part of a whole. Ignoring what's been selected so far can lead to inefficient use of availability constraints, potentially leading to higher capacity than we need
- #1384 Zero shadow prices can lead to negative activity coefficients which can prevent assets which would probably be feasible from dispatching (see also #992)
- General indecision about divisible assets (which, to remind, mostly exist for the purposes of allowing partial decommissioning). Should we have a "parent" asset made up of lots of discrete units with the pre-defined capacities, something that allows for more continuous capacities (#1269), or scrap this completely and just have a collection of independent smaller assets. What should the user interface be?
- #1209 Related, to the above and point 1, the current approach of splitting assets into small units for re-appraisal can lead to many appraisal rounds which can be inefficient. Should we allow appraising multiple units simultaneously?
- #1313 No good solution yet for processes with multiple output commodities (also related to zero shadow prices)

See sub issues below, but also look at any linked pull requests/issues for full context.