Framework must be able to calculate derived variables

[climbfuji]

For expediency when porting physics into CCPP and testing them with various host models, the framework must be able to calculate derived variables in cases where such conversions are unambiguous and reasonable to implement (this is not an open-ended task!).

For example, if potential temperature is not provided, calculate it from temperature and geopotential.

This is a post-capgen-unification task.