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/chassis_mod.mtl

@@ -0,0 +1,11 @@
+# Blender MTL File: 'None'
+# Material Count: 1
+
+newmtl None.002
+Ns 0.000000
+Ka 0.000000 0.000000 0.000000
+Kd 0.640000 0.640000 0.640000
+Ks 0.800000 0.800000 0.800000
+Ni 1.000000
+d 1.000000
+illum 2