Support setting pos and vel of MuJoCo body with 3 slider joints

[juan-g-bonilla]

• Tickets addressed: Closes Support setting pos and vel of MuJoCo body with 3 slider joints #1194
• Review: By commit
• Merge strategy: Merge (no squash)

Description

• Extend MuJoCo body support to cover translational-only bodies without a free joint:
  • Introduce an internal helper areJoints3DTranslation that detects when the first three scalar joints are sliders aligned with x, y, z.
  • Update MJBody::setPosition and MJBody::setVelocity to:
    • Use the free joint when present.
    • Otherwise drive the first three translational scalar joints when the body is a pure 3D translator.
    • Throw an explicit error if neither pattern is available (no free joint and no 3D translational joint chain).
• Add joint inspection helpers on MJScalarJoint:
  • getAxis() to return the rotation axis for hinge joints or translation direction for slide joints.
  • isHinge() to distinguish rotational from translational joints.
  • Mark MJJoint::checkInitialized as const to support these const accessors.
• Update tests and fixtures:
  • Replace the <freejoint/> in test_ball.xml with three orthogonal slide joints (ball_x, ball_y, ball_z).
  • In test_mass_update, set an initial position on ballBody to validate that the position can be set even through the body is not longer <freejoint/>.
• Update release notes.

Verification

• test_mass_update: confirms that the test passes with the 3-slider joint configuration and an arbitrary initial position set through MJBody::setPosition.

Documentation

Update release notes.

[Will-Schwend]

Great PR! It will be excellent to support attitude free motion explicitly.

[schaubh]

Overall looks good. Just one small request to make MJJoint a hyperlink to the associated class in the release notes. Good to go after that change.

[schaubh]

Actually, the unit tests just wrapped up running and I'm getting lots of errors from simulation/mujocoDynamics/NBodyGravity/_UnitTest/test_gravity.py?

[juan-g-bonilla]

Actually, the unit tests just wrapped up running and I'm getting lots of errors from simulation/mujocoDynamics/NBodyGravity/_UnitTest/test_gravity.py?

This was the culprit:

void MJBody::setVelocity(const Eigen::Vector3d& velocity)
{
    if (this->freeJoint.has_value()) {
        this->freeJoint.value().setPosition(velocity);

I'll amend and ping you when everything passes.