Non-negative or positive physical quanties

For entities it appears we have this logic for potentially absent quantities (e.g fillet radii)

Attr|Desc or type
----|-----
IfcWindowLiningProperties.LiningThickness|"If LiningThickness value is 0. (zero) it denotes a window without a lining (all other lining parameters shall be set to NIL in this case). If the LiningThickness is NIL it denotes that the value is not available."
IfcIShapeProfileDef.FilletRadius|OPTIONAL IfcNonNegativeLengthMeasure
IfcSweptDiskSolidPolygonal.FilletRadius|OPTIONAL **IfcPositiveLengthMeasure**

It is not consistent though, so I propose to unify this.

---

For psets it's a rather mixed bag looking at the widths and the heights for example. I'd say dimensions cannot be zero of the physical elements in our domain. We have the request to unify this.

When a value is not available, the property can be omitted or NominalValue can be NIL. What's the semantics of that actually? Never seen it in use, but it's a change made on purpose in 2x3.

Prop|Type
----|----
Pset_CableCarrierSegmentTypeCableLadderSegment.NominalHeight|IfcPositiveLengthMeasure
Pset_KerbStone.NominalHeight|**IfcNonNegativeLengthMeasure**
IfcMechanicalFastener.NominalLength|OPTIONAL IfcPositiveLengthMeasure
Pset_CourseCommon.NominalLength|**IfcNonNegativeLengthMeasure**

---

Also for newly added properties that are actually physical quantities. Shouldn't they be that: Quantity Sets? (e.g Pset_CourseCommon is new in 4x3 I think).