Blurb::
Use the Surfpack version of Gaussain process as the discrepancy model

Description::
This keyword specifies a generic version of the Gaussian process implemented
in the surface-fitting library called Surfpack. With the exception of the
\c correction_order specification, all default options are used in the 
approximation of the model discrepancy function. That is, DIRECT is used as
the optimization method for finding the maximum likelihood estimates of the
hyper-parameters that govern the trend and correlation functions, the 
correlation lengths are optimized by Surfpack, and ill-conditioning is 
handled internally; gradient-enhanced kriging is not provided as an option in
this application.

By selecting this option, each discrepancy function will take the form of
a Gaussian process
