change order of preface and toc in pdf export

Author: sstrahl

PCPT_01_getstarted.ipynb

@@ -106,7 +106,7 @@
      "output_type": "stream",
      "text": [
       "------------------------------------------------------------------------------\n",
-      "https://github.com/meinardmueller/PCPT/releases/download/v0.6.0/PCPT_0.6.0.zip\n",
+      "https://github.com/meinardmueller/PCPT/releases/download/v0.6.1/PCPT_0.6.1.zip\n",
       "------------------------------------------------------------------------------\n"
      ]
     }

PCPT_02_classes.ipynb

@@ -432,7 +432,7 @@
      "output_type": "stream",
      "text": [
       "__dict__ for class:\n",
-      "{'__module__': '__main__', '__doc__': 'Circle class', 'pi': 3, '__init__': <function Circle.__init__ at 0x7d329022b9c0>, 'area': <function Circle.area at 0x7d329022bf60>, '__dict__': <attribute '__dict__' of 'Circle' objects>, '__weakref__': <attribute '__weakref__' of 'Circle' objects>}\n",
+      "{'__module__': '__main__', '__doc__': 'Circle class', 'pi': 3, '__init__': <function Circle.__init__ at 0x7db50ae3b9c0>, 'area': <function Circle.area at 0x7db50ae3bf60>, '__dict__': <attribute '__dict__' of 'Circle' objects>, '__weakref__': <attribute '__weakref__' of 'Circle' objects>}\n",
       "\n",
       "__dict__ for instance:\n",
       "{'radius': 3}\n",