replaced last occurrences of dh parameters with real values

I would prefer getting this correct from the kinematic parameters and the offsets,
however this will go into another MR

ur_description/urdf/ur10.urdf.xacro

@@ -147,8 +147,8 @@
           <mesh filename="package://ur_description/meshes/ur10/collision/upperarm.stl"/>
         </geometry>
       </collision>
-      <xacro:cylinder_inertial radius="0.075" length="${-a2}" mass="${upper_arm_mass}">
-        <origin xyz="0.0 0.0 ${-a2/2.0}" rpy="0 0 0" />
+      <xacro:cylinder_inertial radius="0.075" length="${0.612}" mass="${upper_arm_mass}">
+        <origin xyz="0.0 0.0 ${0.612/2.0}" rpy="0 0 0" />
       </xacro:cylinder_inertial>
     </link>
 
@@ -182,8 +182,8 @@
           <mesh filename="package://ur_description/meshes/ur10/collision/forearm.stl"/>
         </geometry>
       </collision>
-      <xacro:cylinder_inertial radius="0.075" length="${-a3}" mass="${forearm_mass}">
-        <origin xyz="0.0 0.0 ${-a3/2.0}" rpy="0 0 0" />
+      <xacro:cylinder_inertial radius="0.075" length="${0.5723}" mass="${forearm_mass}">
+        <origin xyz="0.0 0.0 ${0.5723/2.0}" rpy="0 0 0" />
       </xacro:cylinder_inertial>
     </link>