Blurb:: 
(Experimental) Specify the \c fitness_metric used to select the next point

Description::
\c adaptive sampling is an experimental capability that is not ready
for production use at this time.

The user can specify the \c fitness_metric used to select the
next point (or points) to evaluate and add to the set. 
The fitness metrics used for scoring candidate points include: 
\li \c predicted_variance
\li \c distance
\li \c gradient

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