opottemppmdiff needed in ocean_model_z rather than ocean_model

[StephenGriffies]

We need to save opottemppmdiff as part of ocean_model_z rather than ocean_model. We presently save osaltpmdiff correctly on ocean_model_z.

Do you agree?

MOM6-examples/ice_ocean_SIS2/OM4_025/diag_table.MOM6

Line 482 in 7e5d936

"ocean_model", "opottemppmdiff", "opottemppmdiff", "ocean_annual", "all", "mean", "none",2

[StephenGriffies]

Specifically, this diagnostic fix is for the p5 version of OM4. @jkrasting please note this change.