Remove ETSource attribute lists

@ChaelKruip This pull request removes explicit attribute lists from Gquery, Input, Preset, Area, and Carrier, instead using the list of attributes from the Atlas classes.

One exception is the area attribute "[cold_network_potential](https://github.com/quintel/etengine/blob/2f9d5949456aeda87c9324193403fa5f50c6d9a4/app/models/qernel/area.rb#L10)" which does not exist in Atlas. It is, however, used [to determine the max_value for a buildings cooling node](https://github.com/quintel/etsource/blob/master/inputs/demand/buildings/buildings_cooling/buildings_cooling_collective_cooling_network_electricity_share.ad#L11). The input in question [does not appear to be used](http://pro.et-model.com/scenario/demand/buildings/cooling) in ETModel. Perhaps it – and the "cold_network_potential" attribute – can be removed?