Develop an explicit section of "drivers"

We need a new section to cover "drivers", like population or gdp, which can have direct or indirect impacts on emissions. The GDP trajectory in particular should be linked to the variable returned from MACRO to ensure consistency.

Begin by locating the model's existing sections and the variable returned from MACRO. Review how population and GDP are currently represented, then clarify how the new drivers section should connect them to emissions; no file or test is named, so completion criteria need confirmation.

Written by the indexing model from the issue text.