ENH: Support for patching conventions.

This allows iris to handle patching of the conventions global attribute via a defined `cf_patch_conventions` function.
Would you consider replacing `CF-1.5` with `iris.fileformats.netcdf._CF_CONVENTIONS_VERSION` ...

Infact, don't you think that `_CF_CONVENTIONS_VERSION` should be public i.e. `CF_CONVENTIONS_VERSION` ?