Collision: add optional density parameter to API

[scpeters]

🎉 New feature

Quick API change in support of #1329

Summary

This adds a std::optional<double> parameter to override the density in Collision::CalculateInertial to support the //link/inertial/density parameter. This is submitted as a quick API change before the Harmonic release without the implementation of the parameter. If we don't merge this, then we can add an overloaded Collision::CalculateInertial method afterwards.

cc @jasmeet0915

Test it

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 #1330 (00bff48) into sdf14 (2ba6470) will not change coverage.
The diff coverage is n/a.

❗ Current head 00bff48 differs from pull request most recent head 445d3bd. Consider uploading reports for the commit 445d3bd to get more accurate results

@@           Coverage Diff           @@
##            sdf14    #1330   +/-   ##
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  Coverage   87.47%   87.47%           
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  Files         134      134           
  Lines       17751    17751           
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  Hits        15528    15528           
  Misses       2223     2223           

Files	Coverage Δ
src/Collision.cc	98.26% <ø> (ø)

[jasmeet0915]

Thank you for the changes @scpeters. I would also like to point out that a link-level <auto_inertia_params> element (//link/inertial/auto_inertia_params) was also added in #1304 for similar reasons as the link-level <density> element (this is also mentioned in the proposal). So should we further modify the API to take an optional sdf::ElementPtr for the same?
cc: @azeey @quarkytale

[azeey]

I would prefer to wait until after Harmonic to add an overload that also takes into account auto_inertia_params.

[scpeters]

we will add an overloaded API after the harmonic stable release and include the implementation at that time