ACTS-401_nestedSubDetectors

This merge request allows more flexibility in the translation from DD4hep to ACTS geometry. With this merge request Barrels, the Beampipe or a compound structure (containing more than one volume of one hierachy level e.g.{neg endcap, barrel, pos endcap},{neg endcap,pos endcap}) do not need to be direct daugthers of the world volume anymore but can be nested in other subdetectors. The same is allowed for compound structures: endcaps and barrels do not need to be direct daughters of the compound detector anymore, but can be nested. Fixes ACTS-401. Depends on !370 (merged) and should be merged afterwards.