Warning for unknown args with tests

[mondus]

Fixes #931.

Testing for previous behaviour is tricky as it causes a system exit. There are exit tests available in google test but these don't support "test for no exit". It would be possible to have a friend class test the arg parsing function specifically however an exit from google test would not constitute a pass (it just wouldn't fail as the testing would exit with an error code) so this seems unnecessary.

[Robadob]

Can't see anything wrong with it.

[ptheywood]

This needs a way to suppress the warning messages via a config property really to close the issue, as otherwise it just adds unsuppressable warning spam to any models wich just want to add to the default CLI (e.g. the use case which led to me opening the issue).

This doesn't need to be done prior to a release however, but either needs the PR changing to not automatically close the issue, or a new issue opening about suppressing the warning.

Otherwise it does allow the intended use case to work (passing a custom --population N arg to the extreme agent scale test model I wrote, while still allowing -s etc to be used.)

[mondus]

Added a CLI option to suppress unknown args to avoid opening an new issue.

[mondus]

Corresponding docs PR