Plot cube over OS map

[rhattersley]

Scenario:

• load data
• setup current map projection as osgb (with region taken from cube)
• plot OS map image (with region taken from current map)
• cell-plot data

Hypothetical example code:

cube = iris.load("my_file")
iris.plot.map_setup("osgb", cube)
iris.plot.draw_wms(wms_url)
iris.quickplot.contourf(cube)

[bblay]

Depends on #15?
Depends on owslib (or alternative), requested in #66.

[rhattersley]

Now that we're using cartopy (#119) and have support for the OSGB projection this is just a cartopy issue.

[pelson]

this is just a cartopy issue.

Solved in SciTools/cartopy#36, if you have the appropriate data.
Note: it has not been achieved using wms, which is another feature that is ripe for Cartopy enhancement.