Add fluid added mass to inertial

[chapulina]

🎉 New feature

• Part of Hydrodynamic added mass gz-sim#1462

Summary

Added fluid added mass support to the inertial class, as described on the Fluid Added Mass Proposal.

I also had to add addition operators to Matrix6, because I forgot the one API I needed when I created the class in #455 🤦🏽

Test it

Take a look at the new tests.

TODO

• The current implementation doesn't take into account the inertial pose's orientation. I think this may be a mistake. I'm going to revisit this as I open downstream PRs.
• Python bindings

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.

🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸🔸

[codecov]

Codecov Report

Merging #459 (2ad032f) into main (155059b) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main     #459   +/-   ##
=======================================
  Coverage   99.69%   99.69%           
=======================================
  Files          75       75           
  Lines        6863     6887   +24     
=======================================
+ Hits         6842     6866   +24     
  Misses         21       21           

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

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[scpeters]

I clarified one of the new test comments in 2ad032f and from there it looks good to me!