[CF-metadata] hybrid height

[Jonathan Gregory]

Dear John

> I havent seen any example files that use that.
> It would be great to not only make it right, but have an example file that 
> uses it, that we could test software against.

There are examples of this in the UKMO HadGEM1 model, from which files have
been put in the PCMDI archive. I would hope that we'll be able to provide
some example files once we have a person in post to do the work.

Best wishes

Jonathan