Isolate "case generation" piece of UFS_IC_generator.py

[dustinswales]

The core of the replay generation, going from a 2D/3D model state to DEPHY formatted forcing files, should be abstracted and referenced by the UFS replay script.
Isolating the model specific part will allow for other hosts to create their own replay for their respective models.
(Maybe we do this with some other model output to demonstrate?)