Fixed the ddsim tracking regions.

This allows us to use the min Energy cut:

```
  --part.minimalKineticEnergy PART.MINIMALKINETICENERGY
                        MinimalKineticEnergy to store particles created in the tracking region
```

topside/topside_defs.xml

@@ -11,12 +11,6 @@
     <constant name="world_y" value="world_side"/>
     <constant name="world_z" value="100*m"/>
 
-    <comment>
-      For DDsim
-    </comment>
-    <constant name="tracker_region_zmax" value="10*m"/>
-    <constant name="tracker_region_rmax" value="2*m"/>
-
 
     <comment>
       ------------
@@ -273,6 +267,12 @@
       --------------------------
     </comment>
 
+    <comment>
+      For DDsim. See ddsim --help for details.
+    </comment>
+    <constant name="tracker_region_zmax" value="SiliconTrackerOuterBarrelLength/2.0"/>
+    <constant name="tracker_region_rmax" value="SiliconTrackerOuterRadius"/>
+
 
 
   </define>