Include exchange techs in the reserve margin constraint

[SutubraResearch]

Prior to this commit, the reserve margin constraint for region R1 was defined over all technologies in R1 belonging to the tech_reserve set. Exchange techs, e.g. electricity transmission, however, are indexed over region pairs, e.g. R1-R2, and are therefore not included in the current constraint.

This commit exptends the constraint to also include exchange techs.

First, the capacity credit of exchange techs are explicitly accounted for in the constraint. For R1, this is done by searching through all technologies in the tech_reserve set with region XX-R1, where XX is any other model region. This uses the temoa convention that exchange techs belonging to region R2-R1 import commodities into R1 and exchange techs belonging to region R1-R2 export commodities out of R1.

Second, the activity of exchange techs are explicitly accounted for in the determination of the total generation in each time step. Electricity imports are added to the total generation and electricity exports are subtracted from the total generation. This gives a better picture of the net regional generation, which temoa uses as a proxy for regional electricity demand.

[SutubraResearch]

I've accidentally added several unrelated commits to this PR. Will close and resubmit separate PRs.