[CF-metadata] new variables

[Jonathan Gregory]

Dear Christof

> friction_velocity    (m s-1)
This is a general concept and I think it would be useful to be more specific
by specifying which interface and which medium it applies to. For instance
it could be called
  friction_velocity_in_atmosphere_boundary_layer
or
  surface_friction_velocity_in_air
if that's what you have in mind.

> upward_latent_heat_flux_in_air (W m-2)
is already in the table as
surface_upward_latent_heat_flux

> upward_sensible_heat_flux_in_air (W m-2)
is already in the table as
surface_upward_sensible_heat_flux

> water_vapor_mixing_ratio (g kg-1)
is in the table as
humidity_mixing_ratio
but it's kg kg-1 i.e. unit is 1.

> distance (km)
Can you be more specific about what distance you mean?

Cheers

Jonathan