Add rover ackermann model (PX4#46)

* added rover ackermann model

* added rover world and removed default world changes

models/rover_ackermann/LICENSE

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+BSD 3-Clause License
+
+Copyright (c) 2022, Rudis Laboratories
+All rights reserved.
+
+Redistribution and use in source and binary forms, with or without
+modification, are permitted provided that the following conditions are met:
+
+1. Redistributions of source code must retain the above copyright notice, this
+   list of conditions and the following disclaimer.
+
+2. Redistributions in binary form must reproduce the above copyright notice,
+   this list of conditions and the following disclaimer in the documentation
+   and/or other materials provided with the distribution.
+
+3. Neither the name of the copyright holder nor the names of its
+   contributors may be used to endorse or promote products derived from
+   this software without specific prior written permission.
+
+THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS"
+AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
+IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE
+DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT HOLDER OR CONTRIBUTORS BE LIABLE
+FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
+DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR
+SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
+CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY,
+OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.

models/rover_ackermann/model.config

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+<?xml version="1.0"?>
+<model>
+  <name>rover_ackermann</name>
+  <version>1.0</version>
+  <sdf version="1.8">model.sdf</sdf>
+  <author>
+    <name>Benjamin Perseghetti</name>
+    <email>bperseghetti@rudislabs.com</email>
+  </author>
+  <author>
+    <name>Christian Friedrich</name>
+    <email>chfriedrich@ethz.ch</email>
+  </author>
+  <description>Generic RC ackermann rover. Based on 'A model of the NXP Mobile Robotics Buggy3' by Benjamin Perseghetti.</description>
+  <license>
+    <file>LICENSE</file>
+  </license>
+</model>