atm nx and ny attributes are incorrect for single-column runs

[billsacks]

What happened?

When running a single-column case, the atm nx and ny attributes passed to CMEPS are incorrectly set to global sizes rather than 1.

What are the steps to reproduce the bug?

This is observable in case2 of SCT - e.g., SCT_D_Ln7.ne3_ne3_mg37.QPC6.derecho_intel.cam-scm_prep_c6 or SCT_D_Ln7.ne3_ne3_mg37.QPC5.derecho_intel.cam-scm_prep. Observe the dimension sizes of the cpl hist file: the nx is 488 for atm dimensions, when this should be 1. (The first grid cell is filled with the actual data.)

What CAM tag were you using?

cam6_4_173

What machine were you running CAM on?

CISL machine (e.g. derecho)

What compiler were you using?

Intel

Path to a case directory, if applicable

/glade/derecho/scratch/sacks/cesm3_0_alpha09d_fixscol_0715a/SCT_D_Ln7.ne3_ne3_mg37.QPC6.derecho_intel.cam-scm_prep_c6.C.20260716_092504_k4ahu7/run/case2run

Will you be addressing this bug yourself?

Yes

Extra info

As far as I know, this doesn't cause any significant problems. But it leads to larger-than-necessary cpl hist and cpl restart files, and could potentially lead to future issues.

I have a suggested fix incoming.