Make the electricity connection conditional in ventilation, HeatingSystem and Occupant.
add fluid ports to the sturcture.
add basic ventilation systems and None-system for heatingsystem and occupants.
correct/update Buildings/Examples/Structure.mo structure.mo
update occupants for new conditional electrical load.
add system to building. Add IDEAS/Interfaces/Examples/building.mo
Forgotten package commit for None heating system.
Merge pull request #124 from open-ideas/Interface

Interfaces updated based on fluid-ports and conditional electrical plugs. The current Merge will give conflicts when pulling ```issue51_BESTEST``` into ```Annex60```, but these will be handled then.