Remove explicit attribute lists

Instead of listing attributes in Gquery, Input, Preset, Area, and
Carrier, instead use the attribute list from the Atlas models. Now it is
only necessary to add a new attribute to Atlas, and not in both
projects.
Remove ununsed Scenario#area_input_values
Remove Qernel::Area#cold_network_potential attribute
Merge pull request #694 from quintel/simplify-attributes

Remove ETSource attribute lists