Implement nested volumes

[AlvaroEzq]

To address issue rest-for-physics/geant4lib#136.
The rest-for-physics/geant4lib#144 of geant4lib needs to be merged before this one.

Changes of this PR are mainly on the TRestGeant4GeometryInfo::PopulateFromGeant4World function, which is implemented in DetectorConstruction.cxx. These changes are:

• Before, only the direct children of the world were inspected. Now, a recursive function is defined to traverse the geometry from the world down to all the nested volumes defined in the geometry.
• Before, the volume names from Geant4 and gdml were matched by assuming they were process in the same order in TRestGeant4GeometryInfo::PopulateFromGeant4World and TRestGeant4GeometryInfo::PopulateFromGdml. Now, the path of mother-to-daughter is obtained for each geant4 physical volume and converted to the corresponding gdml physical volumes mother-to-daughter path naming (more details in Implement nested volumes geant4lib#144).
• The DetectorConstruction::GetPhysicalVolume function has been updated to account for the new nested volumes structure where children volumes share the same geant4 name (and G4VPhysicalVolume object) across all the mother implementations.

[lobis]

LGTM!

[AlvaroEzq]

Thanks but I will wait a little bit to do some deeper testing and make sure I don't change the behaviour of simulations with no nested volumes.

[lobis]

Thanks but I will wait a little bit to do some deeper testing and make sure I don't change the behaviour of simulations with no nested volumes.

Next time open the PR as draft!