[CF-metadata] standard name proposal for CCMVal

[Jonathan Gregory]

Dear Alison

> In Lagrangian case would you need to have an axis which indexed the
> different parcels?

I haven't used a Lagrangian model either but yes, I think you would have such
a dimension, which would be collapsed to form the mean. We don't have a
standard name for a parcel dimension, but no doubt it can be introduced when
required.

Best wishes

Jonathan