Renamed co2_free_factor method to co2_factor
Renamed co2_free to free_co2_factor
Renamed part_ets into takes_part_in_ets
Pointing to new atlas
Fix to call the correct method in primary_co2_emission

ETEngine automatically appends "_factor" when calling
primary_demand_with, so instead of calling "co2_factor", just
calling "co2" suffices.

Updates Atlas to quintel/atlas@b555020
Fix co2_per_mj_factor on account of the new name for co2_free
Merge pull request #679 from quintel/ets_rename

Ets rename