Put required options into their own argument group

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Right now, all CLI options (whether required or not) fall under the optional arguments section of the help menu. It would be much preferable if required arguments were singled out into a separate argument group. It shouldn't be too hard to do.

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Options in mutually exclusive groups don't really want to cooperate here. If you put a mutually exclusive group in an argument group, any arguments added to the mutex group don't actually get added to the enclosing argument group. That's a bit of an issue.

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I don't know how to fix this