Proposal to describe parametric counting in cost quantities

[Moult]

An IfcCostItem's CostQuantities may be either:

1. Arbitrarily defined as an IfcPhysicalQuantity
2. Parametrically defined as an IfcPhysicalQuantity that is both referenced by the CostQuantities and a related thing.

The "thing" may be:

1. An IfcProduct, IfcProcess, or IfcResource related using IfcRelAssignsToControl
2. An IfcProcess related using IfcRelAssignsToProcess or IfcRelAssignsToProduct to an IfcProduct described in number (1)
3. An IfcResource related using IfcRelAssignsToProcess to an IfcProcess described in number (1) or (2)

Note: the docs mention a directed flow from Cost->Product->Process->Resource, and it is not clear whether you are allowed to go from Cost->Process->Product, for example.

Notably missing is the ability to parametrically "count" objects.

I propose to add a description that an IfcCostItem's CostQuantity may contain an IfcQuantityCount. If the IfcCostItem controls "things", the IfcQuantityCount may be calculated from the total count of things counted.

Because "things" can be many direct or indirectly assigned things, I believe this should be restricted to only direct assignments. Therefore, only a "thing" defined by definition (1) out of the three possibilities shall be allowed.

Potentially, the Formula attribute of IfcQuantityCount may be used to distinguish between different counting mechanisms.