Add joint_natural_frequency and joint_damping_ratio to GenericJoint.

[ThierryBerger]

• Fix Add the possiblity to set joint_natural_frequency and joint_damping_ratio per joint #676

⚠️ I'm not sure about multibodies, their cfm_coeff and rhs_bias computed from joint_natural_frequency and joint_damping_ratio is shared with all constraint solver methods, so we'd probably need a strategy to conciliate different values into one?

• I'd like to add example for that, but lack intuition on what values to input.

[Ughuuu]

Can you clarify about the open question more?

⚠️ I'm not sure about multibodies, their cfm_coeff and rhs_bias computed from joint_natural_frequency and joint_damping_ratio is shared with all constraint solver methods, so we'd probably need a strategy to conciliate different values into one?

I for one don't use multibodies right now, so I'm tempted to just merge this into my fork to test it, but other than that, does it introduce any issues?