Add addition operator to Matrix6

[chapulina]

🎉 New feature

• Follow up to Add Matrix6 class #455
• Broken off Add fluid added mass to inertial #459

Summary

I had missed the most important API - the added mass has to be added!

Test it

See 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 #469 (026c09e) into ign-math6 (88ee511) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 026c09e differs from pull request most recent head a9add9e. Consider uploading reports for the commit a9add9e to get more accurate results

@@            Coverage Diff             @@
##           ign-math6     #469   +/-   ##
==========================================
  Coverage      99.68%   99.68%           
==========================================
  Files             73       73           
  Lines           6901     6913   +12     
==========================================
+ Hits            6879     6891   +12     
  Misses            22       22           

Impacted Files	Coverage Δ
include/ignition/math/Matrix6.hh	100.00% <100.00%> (ø)

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[ahcorde]

Added some suggestions here #470

• You can merge them here
• we can merge this PR and then retarget the other one

Thanks, merged!