Andrei Gheata

I'll fix the compilation and provide a test for this interface.

With the ROOT master, you should use `TGeoVolume::ResetTransparency(char value)` to set per volume (and not per material transparency). Calling `ResetTransparency()` with no value will use transparency per material (default behavior)....

@sethj, using the master of today I've checked: `test/testRaytracing -gdml_name /home/agheata/modeler/surface/atlas-hgtd.gdml -nrays 1 -debug 1 -ongpu 0 -test-bvh 0 -use_surf 0 -point 23.51934584635, 17.141066715148, 344.45000092904 -direction 0.5784236876658104, 0.8157365000698582, -9.290358099212079e-7` which...

> Thanks for looking @agheata ; the units I posted were cm not mm. Could these be the discrepancy? Likely. Running with: `test/testRaytracing -gdml_name /home/agheata/modeler/surface/atlas-hgtd.gdml -mmunit 0.1 -nrays 1 -debug...

> @agheata Does your build have CUDA enabled? Our CI seems to suggest that, somehow, we no longer see the broken behavior _on CPU_ when CUDA is enabled in vecgeom....