Allowed >f8 data arrays to be saved to pp

Change the way pps are saved so that they can write >f8 data. I suspect it might not be as simple as I'm making out to fix - I couldn't find documentation on how lbuser is set. However, I can now save my data and open it again, so that's a good sign!

Issue first raised here
https://groups.google.com/forum/#!topic/scitools-iris/aWWk_5QpdjI
Seems to happen particularly when saving cubes resulting from some kind of aggregation.