Add LookupWheelSlip system demo

jetty_demo/models/gravel_platform/model.config

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+<model>
+    <name>Gravel platform</name>
+    <version>1.0</version>
+    <sdf version='1.12'>model.sdf</sdf>
+
+    <author>
+        <name>Ian Chen</name>
+        <email>ichen@openrobotics.org</email>
+    </author>
+
+    <description>
+        A gravel platform
+    </description>
+</model>

jetty_demo/models/gravel_platform/model.sdf

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+<?xml version="1.0" ?>
+<sdf version="1.12">
+  <model name="gravel_platform">
+    <link name="link">
+      <pose>0 0 0 0.0 0 0</pose>
+      <collision name="collision">
+        <geometry>
+          <box>
+             <size>10 10 8</size>
+          </box>
+        </geometry>
+      </collision>
+      <visual name="visual">
+        <geometry>
+          <box>
+             <size>10 10 8</size>
+          </box>
+        </geometry>
+        <material>
+          <diffuse>1 1 1</diffuse>
+          <pbr><metal>
+          <albedo_map>materials/textures/gravel_diffuse.png</albedo_map>
+          <normal_map>materials/textures/gravel_normal.png</normal_map>
+          </metal></pbr>
+        </material>
+      </visual>
+    </link>
+  </model>
+</sdf>