Made own use in the energy power sector dynamic

Made own use in the energy power sector dynamic (fixed percentage of converter throughput) by changing the edge into a reversed share and by removing the preset_demand group. Furthermore, the converter is added to the final_demand_group, in line with the cokes oven own use and the refinery own use (see https://github.com/quintel/etsource/issues/725#issuecomment-36336377).