Make argument parsing fail on unknown parameters

[egpbos]

Currently, our custom config class does not fail when it encounters unknown parameters. This is mostly a legacy from the past in which we were using multiple parsing moments. Currently, the parse_unknown_args function is still used to do some complicated help-print delaying. I don't think we still need that either. So let's just switch to parse_args so that the parser automatically checks for faulty arguments.