Use nbval to perform regression testing on MuMoTtest.ipynb

There is already a commented-out command in `tox.ini` for this:
```sh
# Ensure the user manual and regression test Notebooks do not show regressions (do not enable yet)
#pytest --nbval --nbdime TestNotebooks/MuMoTtest.ipynb
```

nbval includes some functionality for selectively comparing saved and computed output for cells (see [the nbval docs for more info](http://nbviewer.jupyter.org/github/computationalmodelling/nbval/blob/master/docs/source/index.ipynb)) but this may or may not be sufficient for performing meaningful regression testing on `MuMoTtest.ipynb` given that:

1. a large proportion of the outputs in `MuMoTtest.ipynb` are graphical
2. nbval does not run Notebooks in a browser so anything requiring Javascript (e.g. ipywidgets) won't work.