Add GDML volume import support

[qmisslin]

Summary

• expose G4GDMLParser in opengate_core
• add a GDMLVolume type to import a complete GDML geometry subtree
• preserve the OpenGATE world and place the imported GDML root inside a selected mother volume
• support GDML setup selection, schema validation, name stripping, and parser overlap checks
• add user documentation and a regression test

Motivation

OpenGATE currently supports exporting geometry to GDML for visualization, but does not provide a native volume type for importing an existing GDML geometry into a simulation.

This change allows externally generated Geant4 geometries to be reused directly inside an OpenGATE geometry hierarchy.

Validation

• clean editable rebuild of opengate_core
• G4GDMLParser availability and construction verified
• GDML solids, materials, logical volumes, and placements imported successfully
• particle transport through the imported geometry verified
• KillActor attachment to the imported GDML subtree verified
• regression test test113_gdml_volume.py passes
• pre-commit hooks pass:
  • trailing whitespace
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  • clang-format

[dsarrut]

You are too fast @qmisslin ;) ! The actions will probably fail because G4 is not installed with the gdml/XercesC but we will look at it.

[qmisslin]

You are too fast @qmisslin ;) ! The actions will probably fail because G4 is not installed with the gdml/XercesC but we will look at it.

Thanks David ! The CI logs seem to confirm your hypothesis.

test113_gdml_volume.py passes on the Ubuntu jobs, but it fails on all Windows jobs (Python 3.11 to 3.14) because G4GDMLParser is not exposed in the Windows opengate-core wheel.

The current Windows build script appears to compile Geant4 without GEANT4_USE_GDML=ON and without an explicit Xerces-C dependency, whereas the Linux and macOS builds already enable GDML.

The other Ubuntu failures come from test111_jobs_run_sequential.py and seem unrelated to this PR.

Would you prefer that we enable GDML/Xerces-C in the Windows build, or that the GDML regression test be skipped when the installed Geant4 does not provide GDML support?