Proposal to split UnitBasis attribute for IfcCostValue into UnitBasis and UnitValue

A unit basis allows you to specify a cost rate. For example, an `IfcCostValue` with Applied Value of 100 and a UnitBasis of ValueComponent = 5 and UnitComponent = an SQUARE_METER IfcSIUnit means that the the cost rate is 100$ per 5m2 of the thing you are costing.

This is great, except that sometimes there is no unit. For example, you can get a cost rate of 100$ per 5 objects, equipment, or whatever thing which is simple "counted" and no measured / dimensional unit.

So the proposal is to change the data type of UnitBasis to IfcUnit (instead of IfcMeasureWithUnit) and the UnitValue to IfcValue. Both will have an optional cardinality. If UnitValue is omitted, it will be assumed to be one.

Ping @myoualid