Add implementation for signal handling. Use generic signal handler in DDG4 for controlled shutdown on CTRL-C. Add test DDG4_SIGINT_handler in examples/DDG4

Add implementation for signal handling. Use generic signal handler in DDG4 for controlled shutdown on CTRL-C. Add test DDG4_SIGINT_handler in examples/DDG4

Too much hassle to ensure that the list was not duplicating the entries in the list

The original implementation also caused infinite loop by appending to the list that was iterated over causing ddsim to use up all the memory and not do anything

Added a test with an example ddsim steering file

Fix DD4hep_DetectorDump plugin to avoid crash. Add test to check if DetElements have proper placement set

Remove support for very old versions of ROOT < 6.26.0. Remove default use of std namespace from some implementation files.

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

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