Investments thrown by zero/questionable shadow prices

[tsmbland]

I've had a look at the failing models, and these are all due to commodities having zero/questionable shadow prices in one year, which makes activity coefficients negative the following year:

• circularity: GASNAT in 2030 has a shadow price of zero in summer because there's no demand. In 2040 there is demand for GASNAT in summer (in fact, there's only demand in summer) but the activity coefficients are negative so this doesn't get met

• two_outputs: GASPRD in 2020 has a shadow price of zero in summer because there's excess supply (it's a side product of OAGRSV). Activity coefficients in 2030 are negative. (EDIT: fixed by Allow asset selection to continue with unmet demand #1365)

• missing_commodity: BIOPEL in 2030 has no consumption/production in summer. The shadow price, while not zero (should it be?) is very small, so the coefficients in 2040 are negative. (EDIT: fixed by Allow asset selection to continue with unmet demand #1365)

These all seem like fairly normal scenarios so makes me less convinced that #1347 should be a special feature hidden behind a warning, unless there's a deeper issue here that we can fix.

In the latter two cases, it invests in assets to meet demand in the other seasons, but then exits when it has leftover demand in summer that it's chosen not to meet with these assets. I think an issue with the approach suggested in #1347 is that it would allow the system to invest in extra capacity to meet the demand in summer, whereas really it would have been better to utilise the existing assets in summer all along.

Originally posted by @tsmbland in #1357 (comment)