Improve carbon accounting in exporter

[lindnemi]

See #52. Some minor issues with the carbon balance in the exporter could not be completly eliminated before publication of the report:

• efuel -> CC should be corrected with oil_fossil_fraction
• small refactor to use co2_stored more consistently for above causes
• add back in the asserts that were disabled to get to exports faster
• Are fossil CC really treated as neutral, or is there more that should be subtracted?
• Figure out remaining difference of 5-10 Mt compared to Ariade accounting, to get to an equal CO2 balance
• Consider Carbon Sequestration in Emissions|CO2 (as well as FOLU?)