Evict large function definitions from the Helpers.hh header file

[jwnimmer-tri]

🦟 Bug fix

Summary

Evict large function definitions from the Helpers.hh header file, per Google Style Guide.

This is a prerequistite for fixing the static storage duration of the std::regex used by the duration parser, towards the goal of #269.

Checklist

• Signed all commits for DCO
• Added tests (N/A)
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing) -- no new errors; the 9 errors on the main branch are also reported on this PR.
• All tests passed (See test coverage)
• While waiting for a review on your PR, please help review another open pull request to support the maintainers

Note to maintainers: Remember to use Squash-Merge

[jwnimmer-tri]

In other projects, sometimes when I move code from hh to cc files, I'll add Co-authored-by lines to the commit message to reflect the original (header file) authorship of the moved lines back into the cc file. Let me know if you'd like me to do that here.

[ahcorde]

Windows is not happy https://build.osrfoundation.org/job/ignition_math-ci-pr_any-windows7-amd64/1332/consoleFull#-19329938068ff58640-3599-4406-a210-216932f1748c

[jwnimmer-tri]

Windows is fixed as of the latest push. I'd missed the IGNITION_MATH_VISIBLE the first time around.

[mjcarroll]

Let me know if you'd like me to do that here.

I don't think it's necessary here.

windows ci is fixed