This fixes runtime errors like:

    libc++abi: __cxa_guard_acquire detected recursive initialization

Allow for XML processing of files relative to xml tag location (See issue https://github.com/AIDASoft/DD4hep/issues/1180 for details)

Co-authored-by: Andre Sailer <andre.philippe.sailer@cern.ch>

Since 1.26 (ab9a96e6) we are seeing `ddsim` exceptions on certain Open Science Grid nodes/sites. Those sites appear to configure the running of singularity containers in such a way that no usernames are associated with the uid.

This is done by actually reading an event instead of skipping one.
Skipping may be sufficient for the ASCII reader, however, RootTree reader does not perform a read on skip:
https://gitlab.cern.ch/hepmc/HepMC3/-/blob/master/rootIO/src/ReaderRootTree.cc#L76-80

Implement correct volume transparency settings for ROOT>=6.29 (https://github.com/AIDASoft/DD4hep/issues/1117)

for some reason, this fixes the nans and error in the test