Documentation of `IfcVoxelGrid`

1. As discussed during the technical calls today, I see two possibilities for improvement of attributes 4-6 (`NumberOfVoxels?`).

https://github.com/bSI-InfraRoom/IFC-Specification/blob/8525234d2b51b1ad8ad2a8b0c9c22f80fa665d33/IFC4x3/Sections/Resource%20definition%20data%20schemas/Schemas/IfcGeometricModelResource/Entities/IfcVoxelGrid/DocEntity.xml#L13-L21

Option 1: Proposal to change the type to `IfcPositiveInteger`.

I can hardly imagine having a negative or zero length dimension in any direction of the array.

Option 2: Proposal to add a Where rule disallowing value of 0 and improve documentation.

A negative value in the `NumberOfVoxels?` would mean that the grid expands in the negative direction of the corresponding axis.

2. Add explicit documentation for the base point of the representation.

The 1st voxel has one of its vertices in the (0,0,0), right? In the case of Option 1 above, this would be left-top-back vertex given the coordinate system placed on a piece of paper with positive x going right, positive y going up, and positive z going towards the observer. For Option 2 above, it becomes a bit more complicated.