Add jaxsim.mujoco package with resources to render a scene using the Mujoco viewer

[diegoferigo]

This PR introduces a new jaxsim.mujoco package for visualizing a simulated trajectory in the Mujoco viewer. The implementation uses the asynchronous (non-blocking) passive viewer, and it allows to both open a window and/or record a video -- assuming a proper configuration to exploit EGL on headless machines.

So far, only kinematics is visualized. A list of missing features that might be implemented in future PRs is the following:

• Populate the scene with additional shapes through the user_scn member.
• In this way, visualize the active contacts and active contact forces.
• Allow to draw lines e.g. CoM trajectory.

Implementation details:

• The implementation is based on ami/rod. We use this library to pre-process and validate the input URDF/SDF. Passing through sdformat has the advantage to fill data that might not be present in the original model description, simplifying the parsing later done by the new jaxsim.mujoco resources.
• We remove the non-controlled joint by lumping their parent/child links together (this is actually done by sdformat).
• Only the visual shapes are visualized. The collision shapes are rendered with alpha=0.

Therefore, we perform: URDF/SDF → in-memory ROD model → export to URDF → load into the Mujoco URDF parser → export the XML → postprocess the XML with additional elements.

# Convert URDF/SDF to MJCF
python -m jaxsim.mujoco --description /path/to/model.{urdf|sdf} --export /path/to/out.xml

# Visualize URDF/SDF
python -m jaxsim.mujoco --description /path/to/model.{urdf|sdf} --visualize

Full commandline:

jaxsim on  feature/mujoco_passive_viewer via 🐍 v3.11.7 🅒 /jaxsim
❯ python -m jaxsim.mujoco -h
usage: jaxsim.mujoco [-h] -d INPUT_FILE [-m NAME] [-e MJCF_FILE] [-f] [-p] [-v] [-b x y z] [-q w x y z]

Process URDF and SDF files for Mujoco usage.

options:
  -h, --help            show this help message and exit
  -d INPUT_FILE, --description INPUT_FILE
                        Path to the URDF or SDF file.
  -m NAME, --model-name NAME
                        The target model of a SDF description if multiple models exists.
  -e MJCF_FILE, --export MJCF_FILE
                        Path to the exported MJCF file.
  -f, --force           Override the output MJCF file if it already exists (default: False).
  -p, --print           Print in the stdout the exported MJCF string (default: False).
  -v, --visualize       Visualize the description in the Mujoco viewer (default: False).
  -b x y z, --base-position x y z
                        Override the base position (supports only floating-base models).
  -q w x y z, --base-quaternion w x y z
                        Override the base quaternion (supports only floating-base models).

For something similar to a tutorial on how to visualize a model, refer to the __main__ file that includes a handy command-line.

Partially addresses #11.

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📚 Documentation preview 📚: https://jaxsim--83.org.readthedocs.build//83/

[flferretti]

LGTM, thanks! I just wrote some minor comments

[diegoferigo]

As a reference, here the result of the visualization of the ergoCubSN001:

export GAZEBO_MODEL_PATH=/home/dferigo/git/ergocub-software/urdf
python -m jaxsim.mujoco -d ~/git/ergocub-software/urdf/ergoCub/robots/ergoCubSN001/model.urdf -v -b 0 0 1