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}
}

setup.py

@@ -548,7 +548,7 @@ hh = setup_py_dir + "/" + datadir
 for root, dirs, files in os.walk(hh):
     for fn in files:
         ext = os.path.splitext(fn)[1][1:]
-        if ext and ext in 'yaml index meta data-00000-of-00001 png gif jpg urdf sdf obj mtl dae off stl STL xml '.split():
+        if ext and ext in 'yaml index meta data-00000-of-00001 png gif jpg urdf sdf obj txt mtl dae off stl STL xml '.split():
             fn = root + "/" + fn
             need_files.append(fn[1+len(hh):])