Allow radiation diagnostics to persist across timesteps

### What is the feature/what would you like to discuss?

Radiation is not run every timestep. As a result, diagnostic values are 0 on non-radiation timesteps. Ideally, per @adamrher , these diagnostics would reflect the last timestep radiation was run (since, particularly with high-res runs, the history frequencies often do not align with the radiation frequencies).

In RRTMGP, most diagnostic fields (see: FLNT) are interstitial variables that are handled by the framework. We'd have to move them to the registry to persist across timesteps.

### Is there anyone in particular you want to be part of this conversation?

_No response_

### Will this change (regression test) answers?

Yes

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this