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

Geant4InputAction: move EventReader creation to begin run, so we have the run info at this point for example

because the ASCII reader does not parse the run info when not reading a full event, we open the file, skip an event (which makes it read the run info), and then close and open again

we can just concatenate the strings like this. or better use textwrap.dedent and multiline strings.