Fix summary amount calculation and add Öresutjämning row

- Amount = round(volume, 2) * round(avg_price, 2)
- Add Öresutjämning showing difference between transaction sum and calculated total

templates/customer.html

@@ -171,6 +171,12 @@
                     <td></td>
                     <td>{{ customer.summary.grand_total }} Kr</td>
                 </tr>
+                <tr>
+                    <td>Öresutjämning</td>
+                    <td></td>
+                    <td></td>
+                    <td>{{ customer.summary.oresutjamning }} Kr</td>
+                </tr>
             </tbody>
         </table>
     </div>