Blurb::
Perform k-fold cross validation
Description::
General k-fold cross validation may be performed by specifying
``cross_validation``.  The cross-validation statistics will be calculated
for all metrics.

Cross validation may further specify:

- ``folds``, the number of folds into which to divide the build   data (between 2 and number of data points) or
- ``percent``, the fraction of data (between 0 and 0.5)   to use in each fold.

These will be adjusted as needed based on the number of available
training points.  The default number of folds k = 10, or 0.1
Topics::
surrogate_models
Examples::

Theory::

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