Fix units and description for rt_diabatic_tend in atmosphere core Registry.xml

[mgduda]

This PR corrects the units and description for the rt_diabatic_tend field in the atmosphere Registry.xml.

The rt_diabatic_tend field represents the tendency of the modified potential temperature due to cloud microphysics in K/s; accordingly, this commit corrects the units and description attributes for rt_diabatic_tend in the atmosphere core's Registry.xml file.

[smileMchen]

This change is correct.

[ldfowler58]

I am wondering if it would make sense to move rt_diabatic_tend from "tend" to "tend_physics" in Registry.xml. My reasoning is twofold:

1. rt_diabatic_tend is actually a physics-related tendency from cloud microphysics processes, and it is an uncoupled variable as the other physics tendencies in "tend_physics".

2. All the variables in "tend" are coupled tendencies but rt_diabatic_tend is not.

Considering that rt_diabatic_tend is only needed in subroutine atm_recover_large_step_variables, it may not be too much work to do this.

Michael, if you are OK to consider it, I am happy to add a PR to include this change. Thanks.

[mgduda]

@ldfowler58 I don't have a strong opinion on where rt_diabatic_tend is stored, but I think you make a good argument for moving it to the tend_physics pool/var_struct. As you said, that would be a separate PR, which would target the develop branch.

For this PR specifically, do the changes to the units and description attributes look good to you?