Add warnings when anelastic approximation is violated

[kaiyuan-cheng]

The reference state is a dry, isentropic atmospheric profile based on the surface potential temperature. Many atmospheric profiles, both realistic and idealized, can deviate significantly from this reference state at certain levels. Should we check whether the anelastic approximation is being violated?

[glwagner]

We could add a diagnostic that is capable of computing this. If I understand correctly, I think we essentially want to compare an estimate of the exact density with the reference density. We could also design a callback that uses this diagnostic to provide a warning. I think just having a diagnostic that users can invoke in a script is a good starting point though.