add missing svg

Signed-off-by: Michael Leuschel <leuschel@uni-duesseldorf.de>

Physics/n_bodies.svg

+<?xml version="1.0" encoding="UTF-8" standalone="no"?>
+<svg 
+   xmlns="http://www.w3.org/2000/svg"
+   height="400" width="400"
+   viewBox="-100 -100 200 200"
+   >
+  
+   
+   <defs>
+    <marker id="arrowhead" markerWidth="10" markerHeight="7" 
+    refX="0" refY="3.5" orient="auto">
+      <polygon points="0 0, 10 3.5, 0 7" />
+    </marker>
+    
+    <marker id="startarrow" markerWidth="10" markerHeight="7" 
+    refX="10" refY="3.5" orient="auto">
+      <polygon points="10 0, 10 7, 0 3.5" fill="green" />
+    </marker>
+    
+    <marker id="endarrow" markerWidth="10" markerHeight="7" 
+    refX="0" refY="3.5" orient="auto">
+      <polygon points="0 0, 10 3.5, 0 7"  fill="green" />
+    </marker>
+  </defs>
+  
+  
+
+    
+</svg>