[CF-metadata] standard names added

[Jonathan Gregory]

The following standard names have been added to the table, following requests
from Knut Lisaeter, Tony Jolibois, Keith Dixon and Burkhardt Rockel, and
corrections have been made to some equivalences with ECMWF GRIB codes, thanks
to Martin Koehler.

Jonathan

sea_ice_x_velocity:m s-1
sea_ice_y_velocity:m s-1
surface_downward_x_stress:Pa
surface_downward_y_stress:Pa
downward_x_stress_at_sea_ice_base:Pa
downward_y_stress_at_sea_ice_base:Pa
ocean_mixed_layer_thickness_defined_by_mixing_scheme:m
ocean_mixed_layer_thickness_defined_by_temperature:m
ocean_mixed_layer_thickness_defined_by_sigma_t:m
ocean_mixed_layer_thickness_defined_by_sigma_theta:m
integral_of_air_temperature_excess_wrt_time:K s
integral_of_air_temperature_deficit_wrt_time:K s
air_temperature_threshold:K
surface_temperature_where_snow:K