[CF-metadata] mixed layer depth

[Jonathan Gregory]

Dear Tony

Bryan Lawrence has pointed out that these quantities should not have different
standard names if they are really the same quantity diagnosed by different
methods. I think they are truly different quantities and so do need different
standard names. Therefore instead of

>   ocean_mixed_layer_thickness_diagnosed_from_mixing_scheme 
>   ocean_mixed_layer_thickness_diagnosed_from_temperature
>   ocean_mixed_layer_thickness_diagnosed_from_sigma_t
>   ocean_mixed_layer_thickness_diagnosed_from_sigma_theta

perhaps another phrase e.g. according_to or defined_by would be a better choice
than diagnosed_from. What do you think?

Cheers

Jonathan