Support reducing a model considering non-zero positions of removed joints

[diegoferigo]

• Extend jaxsim.api.model.reduce with a new joint_position_locked dictionary that specifies the angles of the removed joints. Note that, if passed, it ignores the angles of the kept joints.
• Extended the simple FK operating on a low-level KinematicGraph to support joint angles (it calls the jax function to compute joint transforms).
• Moved the FK operating on a low-level KinematicGraph to a standalone class.
• Removed support for single-axis joint types, now all joints are either fixed, revolute, or prismatic. Since jaxsim.math.joint_model.supported_joint_motion cannot be jit-compiled dynamically over the joint type, it should slightly speed up compilation of RBDAs.
• Now KinematicGraph fully implements the Sequence protocol.
• Update the iDynTree version since the updated test requires Expose std::unordered_map<std::string, double> to SWIG bindings robotology/idyntree#1180.

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📚 Documentation preview 📚: https://jaxsim--137.org.readthedocs.build//137/

[diegoferigo]

Tests are temporarily failing since they need a released version of iDynTree that includes robotology/idyntree#1180.

[diegoferigo]

This PR can be useful to both @flferretti and @YuchuanLu to compute the frame-to-parent-link transform ${}^L \mathbf{H}_F$. As far as I know, they both need to compute the Jacobian of frames rigidly attached to feets (JaxSim provides Jacobians only for link frames). Refer to #139 (comment) for an example.

In the future, we might consider to add a new jaxsim.api.frame module with frame-related logic.

[flferretti]

Thanks Diego, LGTM!