Split off snapshot fields from registry

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

As physics schemes are ported more fields are being introduced into `registry.xml` that are solely for the purpose of reading these fields in from the snapshot (pbuf or tphysac/tphysbc) fields.

To avoid clutter in registry.xml, these could be split off into a separate file, and the generated code for handling those might also potentially be separated into separate modules.

Other notes from CAM-SIMA planning meeting about this:
* @nusbaume pointed out that a snapshot registry may still be needed in the future for single-column CAM applications, as these model runs do not provide the full model state.
* @cacraigucar suggested adding a line in `registry.xml` now to split off the snapshot variables from the rest.

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

@cacraigucar @peverwhee @nusbaume

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

No

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this