Proposal to modify calculation description for IfcCostItem for schedule of rates

The current description for CostValues is:

> Component costs for which the total cost for the cost item is calculated, and then multiplied by the total CostQuantities if provided. **If CostQuantities is provided then values indicate unit costs, otherwise values indicate total costs.** For calculation purposes, the cost values may be directly added unless they have qualifications. Cost values with qualifications (e.g. IfcCostValue.ApplicableDate, IfcCostValue.FixedUntilDate) should be excluded from such calculation if they do not apply.

With the bolded sentence, it talks about two scenarios:

1. CostQuantities exist, therefore CostValues are **unit costs**
2. CostQuantities do not exist, therefore CostValues are **total costs**

However, there is a third possibility. In a schedule of rates, CostQuantities do not exist, but CostValues are still **unit costs**, as they are merely a schedule of rates.

Therefore I propose a new description as follows (changes in bold):

> Component costs for which the total cost for the cost item is calculated, and then multiplied by the total CostQuantities if provided. **If the controlling cost schedule is a schedule of rates, then values indicate unit costs. In other cost schedule types, if CostQuantities is provided then values indicate unit costs, otherwise values indicate total costs.** For calculation purposes, the cost values may be directly added unless they have qualifications. Cost values with qualifications (e.g. IfcCostValue.ApplicableDate, IfcCostValue.FixedUntilDate) should be excluded from such calculation if they do not apply.