Fix doc building (contd.)

Continuation of #290 

- replaced `notebook_sphinxext.py` with the latest version from [yt](http://yt-project.org/docs/3.0/quickstart/index.html).
- modified `notebook_sphinxext.py` to run notebooks with `pylab=True`, `mpl_inline=True`
- removed magic %matplotlib inline` from example notebooks, because it was breaking (not sure why, it was during deepreload)
- fixed bug #293, which was killing one of the notebooks. some values in the count matrix became Nan. They were supposed to be zero, which is the perfect use case for `np.nan_to_num`.