Build PLR within envelope volumes

In ATLAS-P2-RUN4-01-00-00 we use the GeoModelXml "envelope" mechanism to place the PLR detector inside the pixel volume without having to make artificial voids in that volume to fit it.

This has a few consequences for building and simulating the PLR:

• The volume in which PLR will be placed is different depending on whether the ITkPixel is built or not
• If the ITkPixel is present, we need to descend a level deeper than normal in the volume hierarchy to find the "Tree Top" which gets declared to the DetectorManager

Here we now use a "containing detector" to determine if we need to descend into the hierarchy and look for an "envelope volume".

We would also follow the same approach for BCM' eventually.

FYI @npetters @tadej @ncalace @dfellers @spagan @dellacqu @janovak