Fixed imports and exports are not implemented

According to the documentation you can use `base_year_import` and `base_year_export` files to describe fixed commodity imports/exports. However, digging into the code, this data doesn't appear to be used anywhere (except in the legacy sector)

It's worth noting though that it's still possible to model imports and exports as processes, e.g.

Import: IM-OIL -> OIL
Export: OIL -> EX-OIL

Fixed export quantities can then be captured in the preset files (i.e. demand for EX-OIL), and the agent will perform necessary investments in the "process". (See the Qatar model)

Fixed imports can be modeled by setting the existing capacity for the import technologies (and a fixed price projection for the import commodity). This can also allow flexible imports if more commodity is required (i.e. extra investments in the process)

All a bit hacky, but that's how you'd do it.