Make the root_link the father of the base_link instead of the other w…

…ay around

This permits to avoid the Gazebo bug described in #140 . 
Furthermore, this will make sure that even after fixed joint lumping the link in Classic Gazebo will be called "root_link".

simmechanics/data/icub2_5/ICUB_2-5_BB_simmechanics_options.yaml.in

@@ -947,8 +947,8 @@ XMLBlobs:
         <joint name="base_fixed_joint" type="fixed">
           <origin xyz="0 0 0" rpy="0 -0 0" />
           <axis xyz="0 0 0" />
-          <parent link="base_link" />
-          <child link="root_link" />
+          <parent link="root_link" />
+          <child link="base_link" />
         </joint>
   - |
         <link name="l_foot_dh_frame"/>