Adapt to new VecGeom / TGeo2VecGeom
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use full include specifier for VecGeom
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adapt to some VecGeom API changes
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use new TGeo2VecGeom lib for the ROOT<->VecGeom conversions
- this allows to build VecGeom without ROOT, and makes compiling Geant4 with the same VecGeom easier
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cherry-pick commits from John Apostolakis