Stretch 4 MJCF and URDF

The MJCF is now dynamically generated on-the-fly from the stretch4_urdf package using urdf2mjcf and programmatic DOM manipulations.

When the simulator is initialized, get_robot_xml_path() dynamically invokes mjcf_generator.py to convert the appropriate URDF into an MJCF and applies custom modifications required by Mujoco.

Programmatic Transformations Applied

The mjcf_generator.py applies the following changes to the raw urdf2mjcf output:

1. Removes all tags except asset and worldbody.
2. Removes specific tags from worldbody (e.g., light, camera, ground plane).
3. Replaces the default wheel joints with custom bodies (link_left_wheel, link_right_wheel, link_back_wheel) containing class="wheel" and associated wheel rollers to support omnibase motion, and includes omniwheels.xml.
4. Encapsulates lidar and camera geoms with dedicated bodies and includes hemisphere_lidar_cameras_right/left.xml.
5. Replaces link_mast geometry with appropriate collision and visualgeom classes.
6. Renames the root body to base_link and adds an imu site.
7. Encapsulates gripper fingers into link_gripper_slider (if present, e.g. for sg4 tool).
8. Adjusts class="visualgeom" and adds class="rubber" to gripper fingers.
9. Modifies wrist roll to include gripper_cameras.xml.
10. Adds gravcomp="1" to lift, arm, and wrist components.
11. Maps joints to appropriate classes defined in defaults.xml (lift_stretch4, telescope, wrist_yaw_stretch4, etc.).

These steps have been fully automated and replace the previously documented manual processes.