Add dewpoint function to QSat

We should make dewpoint a little function that's part of QSat. That isolates it's complexity in one place, allows for the possibility of unit testing, or testing specific inputs that might fail, and it's a move to better modularization. Right now dewpoint isn't used anywhere else in the code, but it's a basic enough thing that it might be.

This is likely a refactoring that we should do later. But, I did want to bring it up.

_Originally posted by @ekluzek in https://github.com/ESCOMP/CTSM/pull/3688#discussion_r2692750610_

Definition of Done:
- [x] Make dewpoint into a function part of BargroundFluxes
- [ ] Add unit testing for it
- [ ] Move to QSat