Define standard name terms; determine which terms are synonyms

There are a lot of terms used in Standard Names that are not defined, and many of them do not have an obvious definition even in context. In addition, there seem to be many terms that could be synonyms (in which case they should be consolidated), but may have a distinct meaning depending on the context. This issue will list the standard names that need further clarification on what they are supposed to represent.

## Definitions

- `air_pressure_thickness` Originally listed as `pressure_thickness`, this term does not appear in the CF standard names, and only occurs one in CCPP as `pressure_thickness_at_Lagrangian_surface` (with a few more instances of `layer_pressure_thickness`, which appears to be similar or related) but occurs 4 times in the standard names dictionary. With units of Pa listed, it is clearly distinct from the traditional synoptic definition (`thickness` between two pressure layers), but I can not find this term to be in common use, or a proper definition anywhere. "Cloud pressure thickness" appears to be a term used in radiation physics, but it is unclear to me if this is even the same concept, and regardless I was not able to find a good definition for that one either.
- `atmosphere_heat_diffusivity` This term appears often in EDMF contexts, but I have been unable to find a solid/common definition. It appears to be a different quantity to "thermal diffusivity" as defined in the AMS Glossary of Meteorology. It is listed under the CF standard names as "No help available" 😩

## Potential synonyms
- `cloud_condensate` Appears to be a synonym for `cloud_liquid_water`, should we rename the former to the latter? If not a synonym, then we need to document the difference.
- `heat_flux` vs `sensible_heat_flux` vs `temperature_flux`; are these the same thing?

## General definition questions
- Many PBL/SFC flux-related terms are implied to be "vertical flux", is this an assumption we can adopt?