Add in background topography for RTM time slice plot

It seems we can fairly easily add in background topography to `plot_time_slice.py` using the stamen tile option in cartopy. See an example of how to use it here: https://scitools.org.uk/cartopy/docs/latest/gallery/scalar_data/eyja_volcano.html. I suggest we just include this as part of the `hires` option. 