IFC4 - IfcRelSpaceBoundary - IfcInternalOrExternalEnum

Hello,

I faced an issue concerning IfcInternalOrExternalEnum for IfcRelSpaceBoundary. IFC4 has a more complete [IfcInternalOrExternalEnum ](https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2/HTML/link/ifcinternalorexternalenum.htm) for [RelSpaceBoundaries](https://standards.buildingsmart.org/IFC/RELEASE/IFC4/ADD2/HTML/link/ifcrelspaceboundary.htm) than IFC 2x3 :
> INTERNAL | The space boundary faces a physical or virtual element where there is an internal space on the other side.
> -- | --
> EXTERNAL | The space boundary faces a physical or virtual element where there is an external space on the other side.
> EXTERNAL_EARTH | The space boundary faces a physical or virtual element where there is earth (or terrain) on the other side.
> EXTERNAL_WATER | The space boundary faces a physical or virtual element where there is water (water component of terrain) on the other side.
> EXTERNAL_FIRE | The space boundary faces a physical or virtual element where there is another building on the other side.
> NOTDEFINED | No information available.
Currently an IfcRelSpaceBoundary is not exported as EXTERNAL_EARTH when it is underground as for gbXML export or when you look at analytical surfaces :
Best regards.

Trace the IfcRelSpaceBoundary export path and compare how underground boundaries are handled for gbXML export and analytical surfaces. Verify the IFC4 IfcInternalOrExternalEnum mapping; done means underground boundaries are exported as EXTERNAL_EARTH without changing the other boundary classifications.

Written by the indexing model from the issue text.