Tools for plotting UM domains

[nhsavage]

This PR is intended as a way to start a conversation about the code so far. There are really 3 parts:

1. the library for setting up UM domains based on namelists and plotting them on cartopy maps - this is the core of the new module which has been added
2. a test script for the above - I don't know where this belongs, perhaps should just be replaced with doctests to illustrate usage and unit tests
3. an additional setting of code to plot the locations of cities - possibly belongs in the visualisation module