Add general investment variables

[danielhuppmann]

This PR adds variables suggested by @gunnar-pik related to general investment. including adding tiers to already-existing variables.

[FlorianLeblancDr]

Second comment : my problem with the infrastructure variable is that energy supply is part of the infrastruture in some definitions (https://outlook.gihub.org/)

So may be :

• rename as "Non-energy infrastructure" and be explicit about the list of infrastrucutre we are talking about ? (Water, Transport, Télécommunications)

[gunnar-pik]

Having second thoughts about Investment|Energy Efficiency: These are clearly part of Energy Demand, and should therefore be part of the substructure of Investment|Energy Demand|.... For Buildings, for instance, they might be part of retrofits. So I think we can simply leave the high-level Investment|Energy Efficiency variable away

[danielhuppmann]

@gunnar-pik, I'm confused about your comment regarding energy-efficiency investment - this is not part of this PR.

Should I go ahead and merge the two variables in this PR: forestry-investment and infrastructure-investment, or do we need a more in-depth discussion about energy-supply-infrastructure versus other infrastructure?

[gunnar-pik]

You're right, please go ahead with the merge.
We can revisit the issue on energy efficiency investments when we add additional investments for the demand side.