MuonTrackingGeometry - Copy the inert material onto the conditions store DetMgr

Hi everybody,

In the context of ATLASRECTS-6446, we discovered with some pudding that the readout geometry in the Conditions store does not contain the passive material. The solution at that time was simply to use the geometry from the detStore() as the passive volume don't care about the alignment of the active ones. This MR is the attempt to attach the passive nodes to the second readout geomerty. To avoid a deep copy of the corresponding subtrees, I've introduced a GeoPhysVolProxy auxillary class which can be appened to the det mgrs TreeTop. Don't know whether that's the best solution - Tagging: @tsulaia, @boudreau, @rbianchi, @todorova.

@goblirsc, @sroe, @pscholer, @pgadow

Prerequisite to fix the material: ATLASRECTS-7840