Updated refinery efficiency to 0.926 (EU average)

As result of the discussion about the refinery efficiency in https://github.com/quintel/etsource/issues/615, the efficiency of the energy_production_diesel, energy_production_gasoline, energy_production_heavy_fuel_oil, energy_production_kerosene and energy_production_lpg are updated to the EU average value of 0.926. See the *refinery efficiency source analysis*.

This commit does not fix the issue with respect to the disregarded refinery losses for the oil use in the non-transport sectors. A [separate ticket](https://github.com/quintel/etsource/issues/725) is created to cover this issue.