shared.py: _make_backgound_map() and plot_map()

[douglatornell]

Original report by Nancy Soontiens (Bitbucket: nsoontiens, ).

---

These functions could be really useful outside of nowcast, especially when trying to produce figures for papers or talks. Maybe in geo_tools?

This isn't super urgent, I don't think I actually call this anywhere but it was a thought as a read through the module.