[CF-metadata] mixed layer depth

[Jonathan Gregory]

Dear Tony

You're right that the name of ocean_mixed_layer_thickness is rather vague. I
think we could keep this for when the precise definition is unimportant, but
I'd propose we introduce

  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

to be more precise and explicit. Would that be all right? Standard name
parameters could be recorded to indicate the delta-T criterion etc. (We have
agreed that we need a convention for such parameters but it hasn't been framed
yet.)

Best wishes

Jonathan