Use `DIABATIC_FIRST = False`

[dougiesquire]

Our configurations currently use the non-default MOM6 parameter DIABATIC_FIRST = True which means we:

apply diabatic and thermodynamic processes, including buoyancy forcing and mass gain or loss, before stepping the dynamics forward.

We have agreed to change this to the default.

Before changing this for all configs it might be a good idea to check that outputs aren't changed substantially (whatever that means) by changing this.

• [ ] compare test runs (e.g. a few months of MOM6-CICE6/1deg_jra55do_ryf) with DIABATIC_FIRST = True and DIABATIC_FIRST = False and confirm differences in outputs are negligible
• [ ] remove DIABATIC_FIRST = True for all ACCESS-OM3 configs

[aekiss]

We have no choice but to use DIABATIC_FIRST = False, since we want to use a longer tracer timestep, so there's probably no point testing this.