DOC: fix code-block ipython highlighting
I noticed some warnings in the travis doc build log in the sense of "WARNING: Could not parse literal_block as "python". highlighting skipped." 
This was because some of the `.. code-block:: python` directives contained code that could not be interpreted as python. And, additionally, these code blocks were also not highlighted correctly (the `In [1]` prompt were not recognized by the default python highlighter). 

Therefore, I changed those `.. code-block:: python` directives containing IPython prompt to `.. code-block:: ipython` 
