cartopy replaces basemap

[bblay]

Not ready to pull.

Code is now ready for review but many of the images with changed checksums exhibit the bug believed to be addressed in SciTools/cartopy#31. When this bug is fixed, the commit for the image checksums will be updated.

Also, the commit for the image results will be updated (separate repo). You can see them as they are now, here:
bblay/iris-issue-files@15cf122
Many of these images have minor changes to the border lines.

Resolves #15.

[rhattersley]

Wherever this functionality ends up, it should probably by public, not private.

[rhattersley]

The install guide needs updating to reflect the new dependencies.

[pelson]

It would be nice to start a migration guide. Perhaps some of that could be bundled into Cartopy, but the use of iplt.gcm is a Iris specific thing.

[bblay]

Away tomorrow, current status:

• workaround removed for mpl range bug, pending pr (mpl no. 1314, from bblay/pcmrange)
• minor cartopy pr (no. 53, from bblay/margin) required for 0 margins, as before
• there are still image test failures to attend to

Image test failures:

• margins still present for
  • testplot.TestContourf.test_yx
  • testplot.TestContour.test_yx
  • test_quickplot.TestQuickPlotCoordinatesGiven.test_yx
  • test_plot.TestPlotCoordinatesGiven.test_yx
• range bug still present for
  • test_mapping.TestMappingSubRegion.test_simple
  • test_mapping.TestBoundedCube.test_grid
  • test_mapping.TestBoundedCube.test_pcolormesh

(Looking into test_grid, I discovered the transformed points in matplotlib.axes.pcolormesh, line 7505 were all inf)

[bblay]

Moved into #115