fix bug when calling unallocated diagnostics

[JosephMouallem]

Description

The subroutine compute_column_integrated_moles_of_dry_air_and_co2 is called by default to compute:
Diag%column_moles_dry_air_per_square_meter and Diag%column_moles_co2_per_square_meter in GFS_layer/GFS_radiation_driver.F90:

These quantities are not allocated in GFS_layer/GFS_typedefs.F90 if ldiag3d is false in gfs_physics_nml.
Thus, this causes a code crash.
This PR will add an if statement before calling compute_column_integrated_moles_of_dry_air_and_co2

Tested within shield in debug mode with the current public release branches.

Checklist:

Please check all whether they apply or not

• My code follows the style guidelines of this project
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• [] I have made corresponding changes to the documentation
• My changes generate no new warnings
• [] Any dependent changes have been merged and published in downstream modules

[spencerkclark]

Oops sorry @JosephMouallem this was my bad...how do you feel about flipping this (i.e. always allocating these fields in GFS_typedefs.F90) such that one does not need to set ldiag3d = .true. to use the global_mean_co2 diagnostic?

I changed my mind midway through implementing it in #45, and apparently neglected to test it with ldiag3d = .false., leaving it in this semi-broken state.

[lharris4]

@spencerkclark Since this is a 0D and not a 3D diagnostic, the best way is if we don't need to set ldiag3d to get this diagnostic.

[spencerkclark]

Thanks @lharris4—I agree. Sorry for not making my reasoning clearer.

[JosephMouallem]

Thanks @spencerkclark @lharris4 for sharing you thoughts. I moved these fields out of ldiag3d.
@spencerkclark no worries. Please let know if you would like to make any other changes.

[spencerkclark]

Thanks @JosephMouallem—this looks good to me now!

[linjiongzhou]

Hi Joseph and Spencer,

Thanks for your effort on this. It looks good to me.

Linjiong

[spencerkclark]

A gentle ping on this, which I think should be a quick merge.

[lharris4]

@spencerkclark sorry for the delay. I have completed the merge.

[spencerkclark]

All good, just wanted to make sure it didn't get lost. Thanks!