Convert atmosphere + stubs compsets from "F" to "P".

[nusbaume]

There are several compsets where CAM is coupled to only stub components for the surface (e.g. FKESSLER). In order to avoid confusion, it would be better to change the starting letter from F, which usually indicates the existence of a land model and data ocean, with P, which already contains one atmosphere-only compset (PORT).

FYI @billsacks

[swrneale]

Wouldn't this prompt us to move PORT to PPORT then?