add parametric equations for hypotrochoid

docs/build/html/genindex.html

@@ -173,6 +173,8 @@
   </ul></td>
   <td style="width: 33%; vertical-align: top;"><ul>
       <li><a href="time_of_day.html#time_of_day.hours_norm">hours_norm() (in module time_of_day)</a>
+</li>
+      <li><a href="function.html#function.hypotrochoid">hypotrochoid() (in module function)</a>
 </li>
   </ul></td>
 </tr></table>