ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
My thoughts are, patching netcdf file creation without an explicit convention in mind leads to a randomly fudged result from the perspective of other users utilising this file.

Most changes should be done from within iris itself and the site_config comes to its own only when applying an additional conformance checking mechanism tied to an institution.

It may be that you have considered an additional use to the site_config within iris that I have overlooked?