Ordnance Survey WMTS Out of Date

[dchirst]

Description

The Web Tile Retrieval class for Ordnance Survey's map data uses an out-of-date API and so does not work when you try to use it. This class exists in cartopy.io.img_tiles.

OS has a new API service called the OS Data Hub that has replaced the previous API, so we should update cartopy to reflect the new service.

The new class can use the OS Maps API WMS instead.

I'll work on an updated version and submit it for a pull request.