Blurb::
Choose a trend function for a Gaussian process surrogate
Description::
The only trend functions that are currently supported are polynomials.

The trend function is selected using the \c trend
keyword, with options \c constant, \c linear, or \c reduced_quadratic.
The \c reduced_quadratic trend function includes the main
effects, but not mixed/interaction terms.
The Surfpack GP (See \ref model-surrogate-global-gaussian_process-surfpack)
has the additional option of (a full) \c
quadratic.

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