Allow Pset_ConstructionOccurence to also be used on IfcGroup for Systems

[KarinIFCroad]

In Asset Management Registers there is a need to uniquely identify not just the actual individual, but also it’s sub- and main system. By allowing the use of Pset_ConstructionOccurence also on the actual system it belongs to it would give the asset manager the possibility of having one single Pset for identifying it's assets.

Example: An IfcLightFixture AB+2111K=631BA090 Fitted on IfcMember.POST AB+2111K=631PS090 add a few IfcPipeSegment.FLEXIBLESEGMENT and you have a circuit (or a group) AB+21184=631WC001-7 then add to this the Mains cabinet IfcElectricDistributionBoard.SWITCHBOARD _AB+21184=631WC001

They all belong to an IfcDistributionSystem.LIGHTING V=C.VS=QGA.EA001