Implement processor to "undo" region processing

[iamconsortium]

Implement processor to "undo" region processing

[danielhuppmann]

cc @phackstock

[phackstock]

Thanks @danielhuppmann for opening the issue.
My first thought is concerning the case in which the mapping changes between the upload and the download, i.e. potential renaming. Since the model mappings are versioned we could "rewind the clock" and use the one as it was in place when the data was uploaded. Although just because it is technically possible, does not automatically make it a good idea. It might be confusing if the output is different than the current model mapping.

Regarding the open question my preferences would be as follows:

• For doing a "reverse" model mapping (not a complete renaming upon export, which might be another interesting feature), I would keep all the common regions as is and rename the model native ones.
• Implementation my first instinct would be to go with a separate ReverseRegionProcessor as you suggested. The easiest way to avoid lots of code duplication would probably be to inherit from the RegionProcessor and simply overwrite the apply() function.