add Helfand cross-derivatives

[gmao-rreichle]

Currently, the Helfand surface turbulence scheme is used by the AGCM but not by GEOSldas because the scheme is prone to oscillations in off-line simulations.
The Louis surface turbulence scheme provides cross-derivatives (dT/dQ and dQ/dT) that are not provided by the Helfand scheme.
Adding cross-derivatives to Helfand may make it possible to for GEOSldas to use Helfand and thereby synchronize the surface turbulence schemes between the GEOSldas and the AGCM.

Work with the GCM modeling group to add this functionality.
Relevant code is surfacelayer.F90