Harmonize country mappings

[danielhuppmann]

This PR re-implements the region-clean-up (#13), ensuring consistency across R5, R9 and R10 country mappings.

The following design choices were made:

• The Baltic States are part of Europe/EU
• The R9 regions India and China should include only those countries
• The R10 Region India+ and China+ should include neighboring countries that are similar, i.e., on the Indian sub-continent or fit the description "Centrally-planned Asia" (and are close to China)

Of course, these mappings are only a guidance, because actual model implementations will only be an imperfect approximation to these mappings.

[danielhuppmann]

what about all the small countries that aren't listed...

As far as I can see, the only missing countries have less than 1 million inhabitants, mostly small island states. So I would not add them until necessary, in favor of having a concise list.

[danielhuppmann]

Per a discussion with @volker-krey, I added a clear indication that OECD membership is as of the year 1990, to explain why South Korea is not part of the OECD regions. Our preference is to not redefine the regions at this point.

[danielhuppmann]

Re @christophbertram:

PAC_OECD looks to basically be Oceania plus Japan.

Thanks for that observation, the small-island states are actually duplicated in "Other Asia (R10)" - so I removed them from "Pacific OECD".

China+ and India+:

The aim of the R10 regions, as far as I understood, was actually to have aggregrate-regions that are not directly identifiable as a specific country. This is to avoid too much focus on the country-level in IPCC assessments. Hence, I continue that logic (and make minimal changes to previous definitions).