Add rebar-spacing attribute to IfcReinforcingBar for representing a group of rebars

[Aniobodo]

The request

Quoting from the definition here https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2/HTML/schema/ifcstructuralelementsdomain/lexical/ifcreinforcingbar.htm :

A single instance of this class may represent one or many of actual rebars
.

So there is a need for an optional attribute to indicate the spacing, when an instance of IfcReinforcingBar represents a group of rebars.

The image above shows a case where an instance of IfcReinforcingBar represents a group of rebars.

Motivation

More often than not, these rebars grouped together are placed at regular spacing to each other.
The general reinforcement detailing of slabs/walls/plates involves rebar grouping as described above. 

The current problem with IfcReinforcingBar

The single most important attribute missing is the rebar spacing.
Some software vendors try to adapt IfcReinforcingBar to represent rebar grouping, but these rebars lose their identity and attribute once opened outside the origin software. 
Sometimes, it may be possible to get the spacing by some mathematical calculation from the shape representations of the IfcReinforcingBar. But this can lead to significant numeric precision issues; which is not compatible with the industrial practice of using rounded numbers as rebar spacing.

Similar entity in IFC

Each of the longitudinal and transverse directions of standard fabric reinforcement (IfcReinforcingMesh) can be described as a rebar-group; with LongitudinalBarSpacing/TransverseBarSpacing being the spacing (which is missing in IfcReinforcingBar).