Support adding a basemap to network visualisations and animations

I would find it very useful to be able to add a basemap in to the animations created by [`network_anim()`](https://github.com/toruseo/UXsim/blob/832fbefab35f5f01b557a79c1cf33cb6c04be25e/uxsim/uxsim.py#L1715C9-L1715C21). The basemap could be generated by [contextily](https://github.com/geopandas/contextily), and then added to each frame.
