Unused heat

This commit simplifies the heat network from:
![beta_et-engine_com_data_374956_layout](https://cloud.githubusercontent.com/assets/1303760/4792891/e906e552-5de9-11e4-91f2-174977c5d481.png)
to
![etengine_dev_data_latest_layout_edit](https://cloud.githubusercontent.com/assets/1303760/4792899/f97fd6be-5de9-11e4-9806-c252d4c7c37f.png)

Also, the ```energy_heat_network_unused_steam_hot_water ``` is removed from the ```useful_demand``` and ```useful_demand_heat_cold``` converter groups. These groups had no effect because the energy sector (where the node resides) has no need for 'useful demand queries'.

Closes https://github.com/quintel/etsource/issues/866.