Ensure material/volume ID consistency between Geant and VecGeom

[celeritas-project]

Ensure material/volume ID consistency between Geant and VecGeom

[stognini]

Update on ongoing work:

1. Tested 4 different geometries being constructed both programmatically and via the G4GDMLParser (after exporting them as gdml files). This leads to one discrepancy:

• The world volume has its InstanceID defined at its creation time if the geometry is constructed programmatically, but it is always stored last when saved into a gdml file. This results in a shifted list of volume ids when each case is compared.

Not an issue, since materia_ids do not change, and the link volume_id -> material_id is kept in place.

2. Tested loading each one of the aforementioned gdml files using VecGeom's vgdml::Frontend::Load(...) and compared the loaded volume_ids from VecGeom with the expected volume_ids from G4GDMLParser. They match.

I am not sure if the volume_id is the only needed piece for VecGeom's navigator to work, but since the volume_ids match, we should be able to correctly retrieve any needed information:

Get the logical volume_id from a given placed volume using VecGeom; from our imported data at startup, we can now: check what is the material_id associated to that logical volume; use said material_id to fetch the corresponding XS data in the physics tables.

[sethrj]

Currently the geometry (GeoParams and GeoTrackView) return placed ("physical") volume IDs, so we need to change these to unplaced ("logical") IDs. @mrguilima will handle that.