Duplicate installation date property

Pset_ConstructionOccurence and Pset_InstallationOccurrence both have InstallationDate.

Both properties have the same description of "Date on which the element is installed"

Both Psets are applicable to IfcElement.

Proposal 1: remove from Pset_ConstructionOccurence
Proposal 2: merge the psets together

Start by comparing Pset_ConstructionOccurence and Pset_InstallationOccurrence, focusing on their InstallationDate properties, descriptions, and IfcElement applicability. Resolve whether the property should be removed or the property sets merged; the work is done when the specification reflects an agreed decision and no duplicate definition remains.

Written by the indexing model from the issue text.