Several bug fixes in usage, and improvement in usage and help

[rouault]

• Display mutually exclusive arguments as [[-a]|[-b]] in usage

• Add ... trailer to repeatable arguments in usage: [-x]...

• Implement the following enhancements:

By default usage is reported on a single line.

The ArgumentParser::set_usage_max_line_width(width) method can be used to display the usage() on multiple line, by defining the maximum line width.

It can be combined with a call to ArgumentParser::set_usage_break_on_mutex() to ask grouped mutually exclusive arguments to be displayed on a separate line.

ArgumentParser::add_usage_newline() can also be used to force the next
argument to be displayed on a new line in the usage output.

The following snippet

    argparse::ArgumentParser program("program");
    program.set_usage_max_line_width(80);
    program.set_usage_break_on_mutex();
    program.add_argument("--quite-long-option-name").flag();
    auto &group = program.add_mutually_exclusive_group();
    group.add_argument("-a").flag();
    group.add_argument("-b").flag();
    program.add_argument("-c").flag();
    program.add_argument("--another-one").flag();
    program.add_argument("-d").flag();
    program.add_argument("--yet-another-long-one").flag();
    program.add_argument("--will-go-on-new-line").flag();
    program.add_usage_newline();
    program.add_argument("--new-line").flag();
    std::cout << program.usage() << std::endl;

will display:

Usage: program [--help] [--version] [--quite-long-option-name]
               [[-a]|[-b]]
               [-c] [--another-one] [-d] [--yet-another-long-one]
               [--will-go-on-new-line]
               [--new-line]

Furthermore arguments can be separated into several groups by calling ArgumentParser::add_group(group_name). Only optional arguments should be specified after the first call to add_group().

    argparse::ArgumentParser program("program");
    program.set_usage_max_line_width(80);
    program.add_argument("-a").flag().help("help_a");
    program.add_group("Advanced options");
    program.add_argument("-b").flag().help("help_b");

will display:

Usage: program [--help] [--version] [-a]

Advanced options:
               [-b]

[rouault]

FYI: this is the last batch of changes (at least for now) needed to upstream the changes I've done for the GDAL use case

[rouault]

Thanks!