[CF-metadata] cell metrics

[Jonathan Gregory]

Dear Brian

Maybe I'm being a bit too theoretical about this - but it appears to me that
if we don't say "within the grid cell" for every quantity, why should we say
it for area? I feel a bit uncomfortable about making an exception for grid
metrics.

On the other hand, I do agree with you in not liking to make generic quantities
available as standard names. Do you think we should instead have standard names
of e.g. cell_area and cell_thickness? We may indeed need to have standard names
for lots of grid metrics to describe complicated grids.

Cheers

Jonathan