[IMPORT/EXPORT] IFC2x3 files

[pjanck]

File format specification https://technical.buildingsmart.org/standards/ifc/ifc-schema-specifications/

Example files Come together with the format specification.

Reference implementation Many.

Necessary features Support compilation.

Additional context Majority of stuff should be supported already, however, there may be some peculiarities and changes needed to continue supporting 4x1 and 4x3 versions of the standard simultaneously.

This issue is a mental note and serves as a reference to publicly state that OIP does not support IFC2x3 as of yet.