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