add go2 models

.gitignore

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 *.slo
 *.lo
 *.o
-*.obj
 
 # Precompiled Headers
 *.gch

dial_mpc/models/unitree_go2/LICENSE

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+Copyright (c) 2016-2022 HangZhou YuShu TECHNOLOGY CO.,LTD. ("Unitree Robotics")
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+* Redistributions of source code must retain the above copyright notice, this
+  list of conditions and the following disclaimer.
+
+* Redistributions in binary form must reproduce the above copyright notice,
+  this list of conditions and the following disclaimer in the documentation
+  and/or other materials provided with the distribution.
+
+* Neither the name of the copyright holder nor the names of its
+  contributors may be used to endorse or promote products derived from
+  this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

dial_mpc/models/unitree_go2/README.md

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+# Unitree Go2 Description (MJCF)
+
+Requires MuJoCo 2.2.2 or later.
+
+## Overview
+
+This package contains a simplified robot description (MJCF) of the [Go2
+Quadruped Robot](https://www.unitree.com/go2/) developed by [Unitree
+Robotics](https://www.unitree.com/). It is derived from the [publicly available
+URDF
+description](https://github.com/unitreerobotics/unitree_ros/tree/master/robots/go2_description).
+
+<p float="left">
+  <img src="go2.png" width="400">
+</p>
+
+## URDF → MJCF derivation steps
+
+1. Converted the DAE [mesh
+   files](https://github.com/unitreerobotics/unitree_ros/tree/master/robots/go2_description/meshes) to OBJ format using [Blender](https://www.blender.org/).
+   - When exporting, ensure "up axis" is `+Z`, and "forward axis" is `+Y`.
+2. Processed `.obj` files with [`obj2mjcf`](https://github.com/kevinzakka/obj2mjcf).
+3. Added `<mujoco> <compiler discardvisual="false" strippath="false" fusestatic="false"/> </mujoco>` to the URDF's
+   `<robot>` clause in order to preserve visual geometries.
+4. Loaded the URDF into MuJoCo and saved a corresponding MJCF.
+5. Added a `<freejoint/>` to the base.
+6. Manually edited the MJCF to extract common properties into the `<default>` section.
+7. Softened the contacts of the feet to approximate the effect of rubber and
+   increased `impratio` to reduce slippage.
+8. Added `scene.xml` which includes the robot, with a textured groundplane, skybox, and haze.
+
+## MJX
+
+A version of the Unitree Go2 model for use in MJX is available in `scene_mjx.xml` with the following changes:
+
+1. The solver `iterations` and `ls_iterations` were modified for performance.
+2. `frictionloss` was removed.
+3. The friction cone was changed from `elliptic` to `pyramidal`.
+4. All contacts other than the sphere geoms were changed to sphere geoms and placed at the joints positions to simulate collisions at critical locations.
+   - Note: Currently, MJX cannot deal with collision geoms other than spheres. In this case, we have to put many collision spheres in critical locations of the robot such as joints, torso, and feet as illustrated below.
+
+<p float="left">
+  <img src="go2_mjx.png" width="800">
+</p>
+
+## License
+
+This model is released under a [BSD-3-Clause License](LICENSE).
+
+## Acknowledgments
+
+The MJX model of Go2 was contributed by [lonelyfluency](https://github.com/lonelyfluency).