Update URDF parser to use Tesseract XML namespace

[marip8]

This PR addresses #1059 by removing the URDF tag tesseract_version and moving the custom geometry implementations to a tesseract XML namespace. This should allow us to make further Tesseract-specific URDF additions in a clearer manner. Users will need to specify this XML namespace as follows:

- <robot name="my_robot" xmlns:xacro="http://ros.org/wiki/xacro">
+ <robot name="my_robot" xmlns:xacro="http://ros.org/wiki/xacro" xmlns:tesseract="http://github.com/tesseract-robotics">
  ...
</robot>

I think the URL is somewhat arbitrary, but should generally point to a location that gives the user more information about what elements are available in this namespace.

This PR also removes the behavior in tesseract_version < 2 of automatically converting collision mesh files into into convex hulls. Now, if users want tesseract to automatically convert mesh files to convex hulls, they will need to use the <tesseract:convex_mesh file="" convert="true"/> element instead of just the mesh element.

<collision>
  <geometry>
-    <mesh file="" />  <!-- Previously, this mesh was automatically converted to a convex hull under the hood when `tesseract_version` was < 2 -->
+    <tesseract:convex_mesh file="" convert="true"/>  <!-- Now use this element to get the same behavior -->
  </geometry>
</collision>