[0.25deg config] DIAG_COORDS contains both zstar and rho2

[minghangli-uni]

As agreed in the namelist discussion, we want both ZSTAR and rho2 in DIAG_COORDS.

DIAG_COORDS - want both zstart and rho2

However we dont have a proper 0.25deg density coordinate as was noted from ACCESS-NRI/access-om3-configs#40, and further discussions are needed. To clarify, I've summarised some points from that issue.

1. ocean_model_rho2 was used in pan-Antarctic diag_table.
2. It came from https://github.com/COSIMA/mom6-diagnostics/blob/main/PanAntarctic/inputs/RFNC1.ipynb. But it may not be quite what we want, so might be worth talking about how we adjust it.

Since we dont have diag_rho2.nc under /g/data/vk83/experiments/inputs/access-om3, the following proposal outlines how to add ocean_model_rho2 to the MOM input.

NUM_DIAG_COORDS   = 2
DIAG_COORDS       = "z 01 ZSTAR", "rho2 02 RHO"
DIAG_COORD_DEF_01 = "FILE:ocean_vgrid.nc,interfaces=zeta"
DIAG_COORD_DEF_02 = "FILE:diag_rho2.nc,interfaces=rho2"