Move cables and staves in strip barrel into volumes

Assemblies are groups of volumes without a defined bounding solid. When placing multiple assemblies into a volume, only the totality of all their constituents must not overlap. However, when importing into Geant4, assemblies are removed and the placements are duplicated to allow efficient navigation between non-overlapping siblings.

For our case, this essentially led to a completely flattened tree and resulted in LongStripBarrel1 having a total of 21840 daughter volumes. After the two changes in this merge request, the staves are placed into proper volumes and the barrel volumes have up to 150 daughters at most. The new volume with the most daughters is now LongStripEndcapN with 7356 daughter volumes, which will be addressed in future changes.