Add parallel kinematics plugin / prototype

[gkjohnson]

Add prototype support for parallel kinematics even though they're not supported in the URDF 1 spec.

Format

• Use something like <ParallelLink> to mirror <Link> so the files are compatible with other parsers.

Solver

• Use a something like a simple gradient descent solver.
• Don't attempt to limit user-specified joint positions based on parallel links.
• Fixed joints for the solve must be specified -- ie all known joint positions should be fixed.
• Intended for simple, unknown, passive joints in the system.

Existing Solvers

• THREE.CCDIKSolver
• https://github.com/lo-th/fullik
• https://jsantell.github.io/THREE.IK/
• https://github.com/gkjohnson/closed-chain-ik-js/

[gkjohnson]

https://arxiv.org/abs/2504.04767