Returns the value of PrimaryUnit attribute. This attribute represents primary unit assigned to the definition of the property. It should be provided, if the PropertyType is set to:
P_SINGLEVALUE: determining the IfcPropertySingleValue.Unit.
P_ENUMERATEDVALUE: determining the IfcPropertyEnumeration.Unit.
P_BOUNDEDVALUE: determining the IfcPropertyBoundedValue.Unit.
P_LISTVALUE: determining the IfcPropertyListValue.Unit.
P_TABLEVALUE: determining the IfcPropertyTableValue.DefiningUnit.