ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
Not optional. As it stands at the moment if the `site_config.py` has a `cf_profile` but not a `cf_patch` then a `KeyError` exception will be raised. To remedy this use your `get` on the dictionary and raise a warning if the `cf_patch` is not available. This means this will alert the user that monet is not active and that the `site_config.py` is incomplete.