Rename `exner` to `exner_pbl` in CAM-SIMA.

### Issue Type

Documentation Update

### Issue Description

Currently the local CAM-SIMA variable name for `reciprocal_of_dimensionless_exner_function_wrt_surface_air_pressure` is just `exner`. However, this causes a fair amount of confusion, as that quantity is not actually the traditional `exner` definition.

Thus to help with scientific clarity, it would be beneficial to change the variable name from `exner` to `inv_exner_pbl` in CAM-SIMA.

### Will this change answers?

No

### Will you be implementing this yourself?

Any Software Engineer can do this