Add distribution losses for coal gas

[mabijkerk]

Background
In quintel/etlocal#614 it became apparent that transformation of blast furnaces and coke ovens works well for countries like the Netherlands. In other words, countries where coal gas is only consumed by power plants, CHPs, own use of blast furnaces and coke ovens, and final consumption by Iron & steel.

Issue
In other countries however, coal gas also goes to other flows, like final consumption in the non-metallic minerals sector. Because these flows currently can't be mapped, the mapping of transformation of blast furnaces and coke ovens in the graph for affected countries goes wrong.

Solution
Ideally, all flows for coal gas are properly mapped from the EB to the energy graph. In practice, this is a lot of work. One of the major flows for coal gas however are the distribution losses. Adding these losses to the energy_distribution_coal_gas node should be relatively straightforward and should already significantly improve the mapping of blast furnaces and coke ovens transformation.