Revisit several reference pressure-related standard names

There may be redundancy with the following variables:

reference_pressure
reference_pressure_at_surface
base_state_surface_pressure_for_hybrid_vertical_coordinate

Recall that "reference_pressure" is already in the CF standards and is described as "A constant pressure value, typically representative of mean sea level pressure, which can be used in defining coordinates or functions of state."

CF standards also have a variable called "reference_air_pressure_for_atmosphere_vertical_coordinate" which they describe as "For models using a dimensionless vertical coordinate, for example, sigma, hybrid sigma-pressure or eta, the values of the vertical coordinate at the model levels are calculated relative to a reference level. 'Reference air pressure' is the air pressure at the model reference level. It is a model-dependent constant."

None of these are currently used in UFS-based physics, so we'd like to understand how the original 3 variables differ and whether we should use existing CF standard names or try to construct something more specific for CCPP. From my point of view, all 3 of them could be combined into what CF describes as "reference_pressure".

To the extent that all 3 are needed, we need to clarify how they differ and sync up with what CF has.