Some 'index' fields are real instead of integer for CAM snapshot and should be changed to integer when CAM is retired

### What is the feature/what would you like to discuss?

Because of https://github.com/ESCOMP/CAM/issues/1243

Some integer fields in the pbuf are written out as a real when this field is absolutely needed, e.g., in vertical diffusion to get `pbuf_clubbtop` as the CLUBB top index. This means that correspondingly, these fields had to be declared as `real(kind_phys)` in SIMA and atmospheric_physics.

When CAM snapshots are retired,
* SIMA needs to be updated to have infrastructure to read integer type fields in `physics_data.F90`, and
* registry and physics schemes and their metadata need to be updated to change these fields back to `integer`.

### Is there anyone in particular you want to be part of this conversation?

@nusbaume

### Will this change (regression test) answers?

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this