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

### 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).

Start with the linked IFC4 ADD2 IfcReinforcingBar definition and compare it with IfcReinforcingMesh's longitudinal and transverse spacing attributes. Determine how an optional spacing attribute should represent grouped bars and preserve the rounded spacing semantics described in the issue. Done means the IFC specification defines and accepts this attribute.

Written by the indexing model from the issue text.