ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
You also have the option of:
```python
from contextlib import nested

with nested(_patch_site_configuration(),
            self.temp_filename('.nc')) as (_, nc_path):
    iris.save(cube, nc_path)
    res = iris.load_cube(nc_path)