CAM5 UW PBL: update to use atmos_phys_pbl_utils

During review of #370, @nusbaume originally wrote:
> I wouldn't do it for this PR (as it will change answers), but at some point it would be good to see if we can use the functions in atmos_phys_pbl_utils to do these calculations, as it generally seems like pretty standard versions of these fields (except perhaps how they are calculating n2). I should note that this would require PR https://github.com/ESCOMP/atmospheric_physics/pull/347 to be merged in first. Maybe add this to the "future work" issue?