Backport newest appendToStream functions

[chapulina]

🎉 New feature

Part of

• Hydrodynamic added mass gz-sim#1462

Backports the final appendToStream functions after these 2 PRs:

• 👩‍🌾 Prevent -0 with out stream operator #206
• Fix output stream operator #376

Summary

While creating a Matrix6 class, I thought I'd add it to ign-math6, but with the updates that have been made to Matrix[3|4] on main. One of them requires the use of the appendToStream function, which was added to main in #206 and improved in #376.

I'm not backporting the uses of that function that were introduced in main, because they aren't backwards compatible. I'll use the function in the Matrix6 PR that is coming up next.

Test it

Run the added tests.

Checklist

• Signed all commits for DCO
• Added tests
• Added example and/or tutorial
• Updated documentation (as needed)
• Updated migration guide (as needed)
• Consider updating Python bindings (if the library has them)
• codecheck passed (See contributing)
• 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 and edit the commit message to match the pull request summary while retaining Signed-off-by messages.

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[codecov]

Codecov Report

Merging #453 (5c43624) into ign-math6 (b6f9587) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           ign-math6     #453   +/-   ##
==========================================
  Coverage      99.65%   99.65%           
==========================================
  Files             67       67           
  Lines           6392     6400    +8     
==========================================
+ Hits            6370     6378    +8     
  Misses            22       22           

Impacted Files	Coverage Δ
include/ignition/math/Helpers.hh	98.76% <100.00%> (+0.04%)	⬆️

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