Handle missing countries in IEA EWEB data

[khaeru]

As shared by @SiddharthJoshi-Git with @r-aneeque and I, the (closed source) code used to handle the IEA Extended World Energy Balances data for the MESSAGEix-GLOBIOM base model scenarios used in the SSP(2024) / ScenarioMIP project uses special treatment for countries not present in the IEA data.

In particular

• Certain defined operations on existing regional aggregates and country data in the IEA source are used
• For instance, the values for R12_SAS are produced using values for OTHERASIA - (OCEANIA - (AUS + NZL)).

To do: Adjust .tools.iea.web to optionally perform the same operations, so that .model.transport receives matching values.