armem_grasping: adding preposes to grasps

/cc @meixner @hyseni @ucesn_gaukel

source/RobotAPI/libraries/armem_grasping/aron/KnownGraspCandidate.xml

@@ -21,6 +21,10 @@
             <ObjectChild key='pose'>
                 <Pose />
             </ObjectChild>
+
+            <ObjectChild key='prepose'>
+                <Pose optional="true" />
+            </ObjectChild>
         </Object>
 
         <!-- A Grasp set is a set of grasps that need to be executed to grasp an object. The order is important! -->