Plotting utilities

[tomalrussell]

Plotting network risk analysis results:

• the main aim is to pull out really useful/reusable higher-level plotting components, to support analysis and reporting
• possibly also to put lower-level helpers in one place if they're not obviously available elsewhere
• see for example use of Cartopy and Matplotlib to generate weighted line plots at nismod/jamaica-infrastructure
• see also exploratory work in nismod/osm-aqueduct