[CF-metadata] one new standard name

[Jonathan Gregory]

Dear Burkhardt

> Then there is no standard name presently, in case I want to use the 
> integrated value (kg m-2) for the change in atmospheric water content due 
> to advection?
> Any chance to get one in the next time?

Of course, we could define change_in_atmospheric_water_content_due_to_
advection (kg m-2). This would set a precedent by introducing change_in,
and I would very much welcome other people's views on whether this is
a good idea.

Best wishes

Jonathan