Include `regions` column for agent-related input files?

I'm wondering if we might want to allow users to have various properties of agents also vary by region, given that agents can operate in multiple regions.

We have separate input files for:

- Commodity portions
- Search space
- Objectives

I think all of these could sensibly vary by region, too, though there's currently no way to express this. In particular, it strikes me as odd that agents have to be responsible for the _same proportion_ of commodity demand for all regions in which they operate. Why, just because there's an agent that operates in, for example, different EU countries, would we assume it's responsible for the same proportions everywhere? Similarly, I can imagine a scenario where an agent can invest in coal power plants in one region, but not another, even though the power plant process type could operate in both (admittedly, this seems unlikely, but I don't think there's any reason we should _prevent_ users from doing this). I have no idea whether it's sensible for agents to have different objectives in different regions, but, again, I don't see any good reason to prevent users from doing it.

I don't think this is esp high priority, but may be worth considering for future work.