Missing documentation for using `argparse` to parse arguments in options file

[tmadlener]

It is possible to parse arguments via argparse in options files, via

from k4FWCore.parseArgs import parser
# use parser as any other ArgumentParser

This is undocumented at the moment.

[Zehvogel]

• [x] https://github.com/key4hep/k4FWCore/pull/169
• [x] https://github.com/key4hep/key4hep-doc/pull/58