Add GDP variables

[danielhuppmann]

This PR adds GDP variables in response to #53 using the 2010-USD as unit - however, the latest SSP-release (see http://data.ece.iiasa.ac.at/ssp/) used 2017-USD as unit for GDP projections.

Any suggestions/preferences on which one to use? @IAMconsortium/common-definitions-coordination @IAMconsortium/common-definitions-macro-economy

[FlorianLeblancDr]

this issue applies to many economic variable @IAMconsortium/common-definitions-macro-economy

I would suggest to stick to $2010, for two reasons:

• practice will evolve on current projects (PRISMA, SPARCLE, ACCRUE, etc.)
• The base year of the SSP database is not systematically taken in models. A few example: IMACLIM and E3ME uses $2014 from GTAP-10. GEM-E3 uses $2017 from GTAP-11. And teams simply use a GDP deflator to convert economic values in $2010

It is also better to set the reference year for constant economic value in only one place, using a tag. We are working on such a suggestion with the macro-economic subgroup (adding a Currency-Tag)

This will allow specific templates derived from the common one to change this reference year easily for all variables.

[lolow]

I agree that monetary values should be consistent across the same database. Implementing a currency tag could be a solution. However, it should not be used in the same way as other tags, rather as a placeholder for a monetary unit to be defined at database level.

[danielhuppmann]

Thank you for the discussion. The currency tag could be a solution, but would require some changes in the nomenclature package and the database infrastructure.

I suggest to merge this PR and revisit once a different unit than USD_2010 is required in a project.

If two members of @IAMconsortium/common-definitions-macro-economy approve this PR, I will merge.

[danielhuppmann]

Thank you for this quick and productive discussion!