Refactor the goods-trade variable

[danielhuppmann]

Based on an email by @gunnar-pik, I realized that the goods-trade variable introduced in #57 deviated from the NAVIGATE and ENGAGE templates by separating imports and exports. I'm not sure if that distinction is strictly necessary?

Also, I suggest to add the Goods category to clarify that this should not include the energy or emissions-allowance trade (see #147).

[orichters]

I do not know whether renaming would be an option, but every single time I look at trade variables, I have to check the definitions whether they represent net imports or exports, so I wonder whether naming it Net Exports|Goods or Net Exports|Oil would not be preferable.

[danielhuppmann]

Interesting suggestion - the NAVIGATE template had separate Export|... and Import|... variables for agriculture, energy, industry and Trade|... for emissions-allowances and energy carriers. So instead of switching everything to Trade, we could harmonize to separate Export and Import variables...

[orichters]

For REMIND, we cannot separate the two. Basically, we just have Net Exports or, by multiplying with -1, Net Imports. But I just think Trade as a name is ambiguous as you don't know whether it is Net Imports or Net Exports

[danielhuppmann]

@FlorianLeblancDr, given that you added the distinction into Import and Export in #57 (which can't be reported by REMIND), is it ok to revert to just one aggregate variable that follows the logic of previous templates?

[danielhuppmann]

Merging to proceed with the ScenarioMIP-SSP update, please start a new PR if you want to discuss a distinction into imports and exports.

[FlorianLeblancDr]

Sorry, just seeing this, need to get my github notification sunder my eyes.

Thanks for dealing with this Daniel and Oliver.
Most macroeconomic models (including CGE) have exports and imports separately, I will create a new PR being flexible on reporting 'net' as well.