Blurb:: define scaling of statistical metrics when adapting UQ surrogates

Description:: When adapting a UQ surrogate model, e.g., by change in
response covariance matrix or level mappings, this option controls the
use of scaling.  The \c absolute option indicates that there is no
scaling such that the statistical change is compared directly to the
convergence tolerance.  The \c relative option indicates that the
change is measured relative to the current reference value (which
evolves during the refinement), such that the convergence tolerance
controls the relative change.

<b> Default Behavior </b>

The default is \c relative.

<b> Expected Output </b>

<b> Usage Tips </b>

<b> Additional Discussion </b>

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