Convert tutorials to jupyter-notebooks

Jupyter notebooks provide a really nice interface to allow us to put code alongside output as well as plots. It would be good to see the tutorials converted to notebooks - these can be included in py.test runs using [nbval](https://github.com/computationalmodelling/nbval), and included in sphinx docs using [nbsphinx](https://nbsphinx.readthedocs.io/en/0.2.16/installation.html).