DOC: add faq and fix a typo in Operator

doc/source/guide/faq.rst

@@ -0,0 +1,28 @@
+###########################
+Answers to common questions
+###########################
+
+Abbreviations: **Q** uestion -- **P** roblem -- **S** olution
+
+Errors related to Python 2/3
+----------------------------
+
+#. **Q:** I follow your recommendation to call ``super().__init__(dom, ran)``
+   in the ``__init__()`` method of my ``MyOperator``, but I get the following
+   error::
+   
+	File <...>, line ..., in __init__
+		super().__init__(dom, ran)
+
+	TypeError: super() takes at least 1 argument (0 given)
+
+   **P:** The ``super()`` function `in Python 2
+   <https://docs.python.org/2/library/functions.html#super>`_ has to
+   be called with a type as first argument, whereas
+   `in Python 3
+   <https://docs.python.org/3/library/functions.html#super>`_, the
+   type argument is optional and usually not needed.
+
+   **S:** We recommend to include ``from builtins import super`` in your
+   module to backport the new Py3 ``super()`` function. This way, your code
+   will run in both Python 2 and 3.

doc/source/index.rst

@@ -8,22 +8,27 @@ The main intent of ODL is to enable mathematicians and applied scientists to use
 Working with ODL
 ----------------
 
-User
-~~~~
+As a user
+~~~~~~~~~
 
 :doc:`guide/guide`
 
-Developer
-~~~~~~~~~
+As a developer
+~~~~~~~~~~~~~~
 
 :doc:`dev/dev`
 
+Answers to common questions
+~~~~~~~~~~~~~~~~~~~~~~~~~~~
+
+:doc:`guide/faq`
+
 Contents
 ========
 
 .. toctree::
    :maxdepth: 3
-    
+
    guide/guide
    dev/dev
    odl

odl/operator/operator.py

@@ -153,7 +153,7 @@ class Operator(with_metaclass(_OperatorMeta, object)):
         The set this operator maps to
 
     It is **highly** recommended to call
-    ``super().__init__(doma, ran)`` (Note:
+    ``super().__init__(dom, ran)`` (Note: add
     ``from builtins import super`` in Python 2) in the ``__init__()``
     method of any subclass, where ``dom`` and ``ran`` are the arguments
     specifying domain and range of the new