remove pose frame from SDF,

allow plane collision shape in SDF
load the Roboschool stadium.sdf in humanoid_knee_position_control.py

src/BulletDynamics/Featherstone/btMultiBody.h

@@ -142,7 +142,11 @@ public:
 
 	btMultiBodyLinkCollider* getLinkCollider(int index)
 	{
-		return m_colliders[index];
+		if (index >= 0 && index < m_colliders.size())
+		{
+			return m_colliders[index];
+		}
+		return 0;
 	}
 
     //