Merge in 1.6.x

These files don't appear in the files-changed (cos' merge reverted version changes from 1.6.x history)
 * ```lib/iris/__init__.py```
 * ```setup.py```

That just leaves ```lib/iris/tests/unit/cube/test_CubeList.py```, which had test classes added in both parents.  Combined includes and kept both tests.