Default values for arguments should be printed out

When running `python runner.py --help`, I can see the help text but no default values for the arguments. I tried to find out how to show them using [Param](https://param.holoviz.org/), with no success.

[AB#5107](https://innereye.visualstudio.com/60ce1777-00d6-4015-82bc-488a0c00202f/_workitems/edit/5107)