Make DD4hepPlugin more flexible (was ACTS-401)

Original author Julia Hrdinka @jhrdinka

Allow sub detectors (beam pipe, single barrel, single outer end caps) and compounds (mother volume of volumes from one hierarchy (e.g. inner tracker, outer tracker) belonging together e.g. inner barrel + inner endcaps) to be nested  into other sub detectors.

E.g. if a DetElement is declared to  be compound, its subvolumes do not need to be direct daughters anymore (compound->{neg endcap, barrel, pos endcap}), but can be nested in other substructures (compound->{barrel,endcaps->[neg endcap, pos endcap]}).