remove pose frame from SDF,

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

data/stadium.sdf

@@ -48,12 +48,11 @@
 			</inertia>
 			</inertial>
 			<collision name='collision_1'>
-			<pose frame=''>0 0 -0.5 0 0 0</pose>
 			<geometry>
-				
-            <box>
-              <size>100 100 1</size>
-            </box>
+            <plane>
+              <normal>0 0 1</normal>
+              <size>100 100</size>
+            </plane>
           </geometry>
 			  </collision>
 			<visual name='visual'>

data/two_cubes.sdf

@@ -89,6 +89,7 @@
  <static>1</static>
       <pose frame=''>0.512455 -1.58317 0.5 0 -0 0</pose>
       <link name='unit_box_0::link'>
+      
         <inertial>
           <mass>1</mass>
           <inertia>
@@ -102,8 +103,6 @@
         </inertial>
         <collision name='collision'>
           <geometry>
-          <pose frame=''>0 0 -1 0 0 0</pose>
-      
             <box>
               <size>1 1 1</size>
             </box>