Add Matrix6 class

[chapulina]

🎉 New feature

• Part of Hydrodynamic added mass gz-sim#1462
• Alternative to
  • Add MatrixX class #442
  • Fluid added mass / symmetric matrix #447
• Builds on top of Backport newest appendToStream functions #453

Summary

Add a new Matrix6 class to be used to store added mass (see gazebosim/gz-sim#1462).

The class is an extension of the existing Matrix4 class, with these differences:

• It has 2 more rows and columns
• It has the improvements that have been made to Matrix4 on main:
  • 👩‍🌾 Prevent -0 with out stream operator #206 and Fix output stream operator #376
  • Use constexpr for simple static constants #283
  • Tidy up copy, copy-assign, and destructors #293
• It doesn't have any of the APIs that don't apply to a 6x6 matrix, such as conversion to pose, affine transforms, etc.
• It has the bare minimum functionality that's needed by added mass, plus some things that were trivial to implement. Notably, it's missing Determinant and Inverse
• It has a Submatrix function, as requested in Add MatrixX class #442 (comment)
• I did some future-proofing to make it easier to port this to main, by using GZ_ header guards, removing redundant ignition::math:: namespaces, etc.

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 #455 (8e99bdf) into ign-math6 (2b373a5) will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@              Coverage Diff              @@
##           ign-math6     #455      +/-   ##
=============================================
+ Coverage      99.65%   99.66%   +0.01%     
=============================================
  Files             67       68       +1     
  Lines           6400     6614     +214     
=============================================
+ Hits            6378     6592     +214     
  Misses            22       22              

Impacted Files	Coverage Δ
eigen3/include/ignition/math/eigen3/Conversions.hh	100.00% <100.00%> (ø)
include/ignition/math/Matrix6.hh	100.00% <100.00%> (ø)

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[scpeters]

I've made a few minor comments, aside from thinking that the multiplication operator would be better off with for loops, since it's so long.

Also, what do you think about adding a SetSubMatrix(Matrix6Corner, Matrix3) method?

[scpeters]

there may be a performance advantage to manually unrolling the loops, but this is a bit too much; I'd rather just have two for loops here

[chapulina]

I was trying to keep the diff to Matrix4 as little as possible, so I followed the pattern there. But I agree with you that this is very error-prone. See efb0f0d for a more compact implementation which doesn't require allocating the matrix with zeroes and then changing each element one by one.

[chapulina]

what do you think about adding a SetSubMatrix(Matrix6Corner, Matrix3) method?

Good idea, this will probably be useful. Added in efb0f0d

[scpeters]

LGTM with clean CI (there are a few lines over 80 characters)