Added 'ukmo__um_version' cube attribute to contain UM version of PP/FF data.

Supported by pp (and FF) load and save.
See also newer suggestions in https://github.com/SciTools/iris/pull/1141, which could make this neater.

The change messes up a lot of CDL/CML result files, and a few doctest and examples, so I've left those fixes in separate commits for now.