Fix for invisible first_time_step bug in SHiELD

[lharris4]

Jan-Huey discovered a nasty bug in the SHiELD driver: if first_time_step is true, then physics diagnostics are written on EVERY timestep. Also, since first_time_step defaults to true, if it is not explicitly set to false, then the problem occurs too, without any indication what is happening.

Description
Jan-Huey discovered a nasty bug in the SHiELD Model driver in atmos_model.F90 . If first_time_step is true, then physics diagnostics are written on EVERY timestep. This creates far more history file output than we want.

But also, since first_time_step defaults to true, if it is not explicitly set to false, then the problem occurs too, without any indication what is happening. This creates a very obscure issue that is hard to debug.

How Has This Been Tested?
Ran a test without atmos_model_nml::first_time_step specified

Checklist:

• 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
• New check tests, if applicable, are included

[lharris4]

Hi, all. I hadn't realized that these updates had not been merged into atmos_drivers.

@linjiongzhou @kaiyuan-cheng @janhueychen can you check over this and make sure that these changes are the right ones to avoid the bug Jan-Huey discovered?

@thomas-robinson can you approve this?

Thanks,
Lucas

[kaiyuan-cheng]

The changes are correct.