keeping results unchanged: - add 2 future run-time params to select which saturation vapor pressure   formula is used and how post-iteration fluxes are set. - Temporary set these 2 params according to option SOLVE4TEMP_LEGACY; - New piece of code (not used) to update surf. atmos. heat flux F_ia using   the linearized expression to stay consistent with tsurf finding.
keeping results unchanged:
- add 2 future run-time params to select which saturation vapor pressure
  formula is used and how post-iteration fluxes are set.
- Temporary set these 2 params according to option SOLVE4TEMP_LEGACY;
- New piece of code (not used) to update surf. atmos. heat flux F_ia using
  the linearized expression to stay consistent with tsurf finding.

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