Group merge

[phlptp]

This PR adds a mechanism to hide groups but have their options visible as part of the parent.
This works for option group names starting with a '+'

for example

 CLI::App app;

    bool flag = false;
    std::optional<bool> optional_flag = std::nullopt;

    app.add_option("--tester");
    auto *m1=app.add_option_group("+tester");

    m1->add_option("--flag", flag, "description");
    m1->add_option("--optional_flag", optional_flag, "description");

    CLI11_PARSE(app,argc, argv);

will produce help as

Options:
  -h,--help                   Print this help message and exit
  --tester
  --flag BOOLEAN              description
  --optional_flag BOOLEAN     description

instead of

Options:
  -h,--help                   Print this help message and exit
  --tester
[Option Group: tester]
  Options:
    --flag BOOLEAN              description
    --optional_flag BOOLEAN     description

Fixes issue #1034 and a few other past issues or questions

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 100.00%. Comparing base (e4ee3af) to head (27137c6).
Report is 27 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##              main     #1039    +/-   ##
==========================================
  Coverage   100.00%   100.00%            
==========================================
  Files           17        17            
  Lines         4546      4569    +23     
  Branches         0       976   +976     
==========================================
+ Hits          4546      4569    +23     

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[phlptp]

@henryiii unless I hear some objections I am going to merge this on Monday.

[henryiii]

Sounds good. Let me know if there’s anything specific to review. At PyCon currently.

[phlptp]

Mostly if you have any concerns about the notation itself and having slightly different behavior depending on the option group name. I know there have been several requests for this capability and I am a little reticent to add more modifiers and options, so this seemed like a nice solution.

[henryiii]

Especially because I have options that belong together, but are in different groups.

That wouldn't be covered by this?

[phlptp]

Especially because I have options that belong together, but are in different groups.

That wouldn't be covered by this?

It should be. The way the mechanic works is that the option would get associated with a parent group, that could be the main app or a sub_group. So you could tie any number of sub_groups together for help display as part of the main app or in another sub group.