ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
Yes, I initially submitted this PR with a modification of the conventions attribute rather than continuing with the existing behaviour of emptying it, however @esc24 was not in favour of this approach (I should have kept this commit to indicate the path followed in the review).

This is the behaviour:
No cf_patch_configuration: {Conventions: 'CF-1.5'}
Yes cf_patch_configuration: Patched Conventions