Add an argument group for required parsed args

[gerritdm]

This way the --help message will be more clear and won't list the required arguments (e.g. --scenario) under optional arguments, which is the default for flags (--foo or -f), whether they are required or not.

Related to #111.

More background: https://stackoverflow.com/questions/24180527/argparse-required-arguments-listed-under-optional-arguments.

[anamileva]

Is there any way to show the required arguments before the optional arguments?

[gerritdm]

Yes, here are some options: https://stackoverflow.com/questions/39047075/reorder-python-argparse-argument-groups.

I'll get this implemented shortly.

Edit: the preferred answer doesn't work unfortunately because of the way how parsers are combined when you use the parent = [] function. Even if you explicitly create an optional group in the parents, and do that after creating the required group in another parent, for some reason the optional group will still be first (even if I set add_help of the child to false, and manually add it later to an explicitly defined optional group).

There is an easy hacky way but that accesses a private list and has internal dependencies that might not last (see last commit).

[anamileva]

There is an easy hacky way but that accesses a private list and has internal dependencies that might not last (see last commit).

Good enough for now given we document it.