Replace implementation of RobotLink::createInertia that relies on gz_math with an implementation that uses Eigen

[Zyrin]

Description

Replaces the use of gz::math::MassMatrix3d::EquivalentBox in RobotLink::createInertia with an implementation in Eigen similar to what was suggested here: #740 (comment)
This removes the gz_math(_vendor) dependency

This was done because we are building ros2(including rviz2) and gazebo separately and ran into the problem of having two different gz_math(_vendor) libraries/versions.

Is this user-facing behavior change?

No.

Did you use Generative AI?

No.

[jmachowinski]

@ahcorde How can we test this ?

[Zyrin]

@ahcorde Do you have any idea how I can fix these linter errors? Eigen is a C++ library so I do not understand the error message.

[traversaro]

Just to understand, so we are removing the heavily tested EquivalentBox gz-math function with some untested code. That seems a bit more as a workaround for a specific problem of a build setup, rather then something useful per se (mostly as in the future it could be convenient to use other harder to replace gz-math functionality). Just to understand, why can't you build gazebo and ros using the same gz-math version?