Physics driver cleanup

[JosephMouallem]

A cleanup is needed after refactoring the physics driver GFS_physics_driver as in #71 where the said driver is split into two components: GFS_physics_driver_down and GFS_physics_driver_up.

• Given that no more refactoring is needed, local variables not used in these components should be removed.
• Intermediate variable currently under Tbd could be used directly in both routines without the need to copy-in/out when transitioning between these two routines.
• Rename Tbd to a more descriptive name