Eliminate requirement of always having specific humidity

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

There is a requirement in CAM, and for the moment CAMDEN, that specific humidity always be present as an advected constituent. However, for certain scenarios (such as a dry physics suite or an exoplanet simulation) there is no reason to have specific humidity, and thus requiring the tracer increases the computational cost and complexity of those simulations.

Thus it would be beneficial, and improve model flexibility, if specific humidity was treated like all other advected constituents, and only included when a physics scheme required it.

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

_No response_

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

I Don't Know

### Will you be implementing this enhancement yourself?

Any Software Engineer can do this