Handle flow=AVBUNK 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 data with flow=AVBUNK.

In particular:

• For flow=AVBUNK and particular countries, data have negative values.
• To compute the totals used to parametrize the base model, these values are (1) multiplied by -1 and (2) added to other transport-related flows to produce a total.
• This, in effect, assigns international aviation energy use to the countries providing these flows into international bunkers.

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

cf. #230