Where to measure flange thickness in case of sloped flanges

I and U shape profiles specify an optional flange slope, which makes thickness non-uniform, but there isn't a clear indication where thickness is supposed to be measured in case of sloped flange.

@Jesusbill suggests:

> pretty sure it refers to the thickness at the edge of the flange and not at the center, like the parameter t in this [link](http://www.b2bmetal.eu/ipn-inp-beams-european-standard-universal-steel-i-beams-ipn-section--flange-slope-14--properties-dimensions-specifications-din-1025-1-1995)

https://github.com/IfcOpenShell/IfcOpenShell/pull/2680

https://standards.buildingsmart.org/IFC/DEV/IFC4_3/RC1/HTML/schema/ifcprofileresource/lexical/ifcishapeprofiledef.htm

https://standards.buildingsmart.org/IFC/RELEASE/IFC2x3/TC1/HTML/ifcprofileresource/lexical/ifcushapeprofiledef.htm

Read the IFC4.3 RC1 and IFC2x3 documentation for IfcIShapeProfileDef and IfcUShapeProfileDef, then review the discussion and linked pull request #2680. Determine and document where thickness is measured for sloped flanges, with wording consistent across the referenced IFC specifications; the issue is done when the ambiguity is resolved in the specification.

Written by the indexing model from the issue text.