explicitly add models to avoid errors

Author: adivardi

opennav_coverage_demo/world/blank.world

@@ -3,14 +3,45 @@
   <!-- We use a custom world for the pioneer so that the camera angle is launched correctly -->
 
   <world name="default">
-    <include>
-      <uri>model://ground_plane</uri>
-    </include>
-
-    <!-- Global light source -->
-    <include>
-      <uri>model://sun</uri>
-    </include>
-
+    <light name='sun' type='directional'>
+        <cast_shadows>0</cast_shadows>
+        <pose>0 0 10 0 0 0</pose>
+        <diffuse>0.8 0.8 0.8 1</diffuse>
+        <specular>0.8 0.8 0.8 1</specular>
+        <attenuation>
+            <range>1000</range>
+            <constant>0.9</constant>
+            <linear>0.01</linear>
+            <quadratic>0.001</quadratic>
+        </attenuation>
+        <direction>-0.5 0.1 -0.9</direction>
+    </light>
+    <model name='ground_plane'>
+        <static>1</static>
+        <link name='link'>
+            <collision name='collision'>
+                <geometry>
+                    <plane>
+                        <normal>0 0 1</normal>
+                        <size>100 100</size>
+                    </plane>
+                </geometry>
+                <max_contacts>10</max_contacts>
+            </collision>
+            <visual name='visual'>
+                <geometry>
+                    <plane>
+                        <normal>0 0 1</normal>
+                        <size>100 100</size>
+                    </plane>
+                </geometry>
+                <material>
+                    <ambient>0.8 0.8 0.8 1</ambient>
+                    <diffuse>0.8 0.8 0.8 1</diffuse>
+                    <specular>0.8 0.8 0.8 1</specular>
+                </material>
+            </visual>
+        </link>
+    </model>
   </world>
 </sdf>