[Geom] Add auxiliary user-data export in TGDMLWrite

[KoljaFrahm]

This Pull request:

Adds the option of exporting auxiliary of TGeoVolumes in TGDMLWrite
Uses same format as in https://github.com/root-project/root/blob/master/geom/gdml/src/TGDMLParse.cxx

Changes or fixes:

Fixes #20356

Checklist:

• tested changes locally
• updated the docs (if necessary)

This PR fixes #20356

[KoljaFrahm]

I reformatted my commit (clang), do I still have to do so something to start the tests again?

[dpiparo]

Is there a way in which we could test this fix?

[KoljaFrahm]

I described two ways in the issues (Reproducer).
Is this enough or should I describe something in more detail?

[KoljaFrahm]

I am not sure why the ruff code analysis fails for files I haven't changed

[agheata]

Thanks @KoljaFrahm for this addition - reading the auxiliary data via TGDMLParse was only intended so far to provide user access to these data via the volume user extensions, but it is indeed nice to be able to re-export it. See my comment requesting a small correction - otherwise your patch works nicely!

[agheata]

Is there a way in which we could test this fix?

Actually, this is not a fix but a feature, allowing the re-export of GDML auxiliary volume data imported from a GDML file. It could be tested eventually if we make a more general GDML import/export regression test

[KoljaFrahm]

Adjusted and rebased :)