Use of group as an argument is too generic

Would you be open to changing the argument groups to something more specific, such as split-groups? I can put up a PR if you like.

"group" is such a generic term and there are bound to be plugins which conflict.

Start by locating the pytest plugin's argument-parsing entry point and every use of the generic “group” argument. Check whether the proposed “split-groups” name is used consistently and whether tests cover the command-line interface; done means the renamed argument no longer risks generic-name conflicts.

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