Use nbval to perform regression testing on MuMoTtest.ipynb

[willfurnass]

There is already a commented-out command in tox.ini for this:

# 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) 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.