add Laikago quadruped model, thanks to Unitree for the cad meshes.

If you use the Laikago URDF, please add a citation to PyBullet:

@MISC{coumans2018,
author =   {Erwin Coumans and Yunfei Bai},
title =    {PyBullet, a Python module for physics simulation for games, robotics and machine learning},
howpublished = {\url{http://pybullet.org}},
year = {2016--2018}
}

examples/pybullet/gym/pybullet_data/laikago/license.txt

@@ -0,0 +1,13 @@
+URDF model created by Erwin Coumans.
+Meshes derived from Laikago cad model, used by permission of Unitree.
+
+If you use the model, add a citation to PyBullet:
+
+@MISC{coumans2018,
+author =   {Erwin Coumans and Yunfei Bai},
+title =    {PyBullet, a Python module for physics simulation for games, robotics and machine learning},
+howpublished = {\url{http://pybullet.org}},
+year = {2016--2018}
+}
+
+