[CF-metadata] various new standard names

[Jonathan Gregory]

Dear Christiane

> I guess it should be humidity_ratio rather than humidity_mixing_ratio?
Mixing ratio is a common term for humidity, so we should stick with
that, I think. There is a distinction between humidity mixing ratio and
specific humidity.

However I agree with you about ozone. We don't need a mixing ratio term
for ozone, as mass fraction is what is used and we have a name for that.

Best wishes

Jonathan