Support default_iff_used in ArgumentParser

[tradej]

See https://github.com/devassistant/devassistant/blob/master/devassistant/cli/devassistant_argparse.py#L98

[hroncok]

I think importing it from devassistant and making the client dependent on devassistant is a bad idea; so maybe just copy-paste the code?

[tradej]

Yes, copy-pasting seems the preferable solution to me too.

[hroncok]

Do we need this for the proof of concept? It's not in the milestone, but I guess some assistants might need it, right?

[tradej]

We'll be demoing only the nulecule assistant, and it doesn't need it.

[hroncok]

ok