Is basename used in ```assertCML``` and ```assertCDL```? I found one occurrence:

```
$:> grep assertCML * -r | grep basename

$> grep assertCDL * -r | grep basename
lib/iris/tests/unit/fileformats/netcdf/test_Saver.py:            self.assertCDL(nc_path, basename='endian', flags='')
lib/iris/tests/unit/fileformats/netcdf/test_Saver.py:            self.assertCDL(nc_path, basename='endian', flags='')
```

I'm tempted to suggest we simply remove basename from all of these as they are adding more complexity (in documentation at least) than they are worth.