[CF-metadata] lake standard names

[Jonathan Gregory]

Dear Burkhardt

> Can we introduce a parameter called "lake_area_fraction" analogue  
> to "sea_area_fraction" and "land_area_fraction"?

Yes. Let us define a standard name of lake_area_fraction.

We probably need a more general method of encoding cover types e.g. as a
dimension, so that we don't have to introduce a multiplicity of names for
particular land cover types. But for your purposes we are currently OK, I
think. If you can use sea/ocean names for lakes, which ones does that still
leave to be defined from your lake list?

> By the way, what is the difference between "sea" and "ocean". When  
> should I use "ocean" and when "sea" for standard_names?

Existing standard names use "ocean" for large-scale properties (of the medium,
and its dynamics) and "sea_water" for local properties (of the fluid). There is
a corresponding distinction between "atmosphere" and "air".

Best wishes

Jonathan