Splitting code cells

I've split the code cells up in the main tutorial notebook and made sure that it matches up in the solutions. In general, I split at every sub-section of an exercise - i), ii), iii), etc - except where the subsections were part of an indented section e.g. a for loop.

The solutions run through just fine, so I think this is ready to merge.

I also added a comment note to the from __future__ import division statement, to say that it's only relevant to version 2.x

Next up, I will add some more about plt.subplots and slice objects to introductory-material.ipynb.