Potential for overcapacity when using processes with multiple output commodities

[tsmbland]

I thought we already had an issue for this, but turns out not.

As explained in #960 and #963, since the investment algorithm considers only primary commodity outputs, there's potential for commodities produced as side products to have too much capacity.

[tsmbland]

I've suggested the possibility of agents considering multiple commodities simultaneously, which I think could work but would also be quite fiddly.

An alternative might be to repurpose the "balancing" idea introduced for circularities in #999. The idea would be that, like with circularities, we begin by perform investments for each commodity in isolation, followed by a global optimisation covering multiple commodities that allows some degree of refinement to capacities. In the case of circularities. this allows for some capacity enhancement that is required for commodities who's demand profiles are altered by the circularity itself. In the case of processes with multiple output commodities, it would allow the system to trim away any excess capacity that may have been introduced by considering commodities separately in the main investment pass. Could also be useful for trade (see discussions in #523).