Refactor of numerical derivatives in TH

[Hammond, Glenn E]

All,

FYI. I have refactored numerical derivatives in TH to mimic what is done in GENERAL, SCO2 and RT (GIRT) where we store copies of perturbed auxvars without copying and perturbing repeatedly over for each process (accumulation, internal flux, bc flux, source/sink, etc.). The refactor also streamlines the source/sink term a bit. I plan to merge this to the master branch later today. From what I can see, no one other than Jumanah has ongoing development in TH.

Regards,

Glenn