Surface area fraction linear closure

[ilopezgp]

Now that we have a pipeline for parameter calibration, it should be easy enough to improve our parameterization of updraft surface area fraction, which currently is:

a_surf = C

We could improve this model by learning a linear representation of the surface area fraction as a function of information given by SurfaceFluxes, e.g.,

a_surf = C_1 + C_2*LHF + C_2*SHF + ...